Re: [Qemu-devel] [PATCH v3 3/3] vfio-pci: process non fatal error of AER

[Alex Williamson]

On Tue, 25 Apr 2017 23:32:52 +0300
"Michael S. Tsirkin"  wrote:

> On Tue, Mar 28, 2017 at 08:55:04PM -0600, Alex Williamson wrote:
> > On Wed, 29 Mar 2017 02:59:34 +0300
> > "Michael S. Tsirkin"  wrote:
> >   
> > > On Tue, Mar 28, 2017 at 10:12:25AM -0600, Alex Williamson wrote:  
> > > > On Tue, 28 Mar 2017 21:49:17 +0800
> > > > Cao jin  wrote:
> > > > 
> > > > > On 03/25/2017 06:12 AM, Alex Williamson wrote:
> > > > > > On Thu, 23 Mar 2017 17:09:23 +0800
> > > > > > Cao jin  wrote:
> > > > > >   
> > > > > >> Make use of the non fatal error eventfd that the kernel module 
> > > > > >> provide
> > > > > >> to process the AER non fatal error. Fatal error still goes into the
> > > > > >> legacy way which results in VM stop.
> > > > > >>
> > > > > >> Register the handler, wait for notification. Construct aer message 
> > > > > >> and
> > > > > >> pass it to root port on notification. Root port will trigger an 
> > > > > >> interrupt
> > > > > >> to signal guest, then guest driver will do the recovery.  
> > > > > > 
> > > > > > Can we guarantee this is the better solution in all cases or could
> > > > > > there be guests without AER support where the VM stop is the better
> > > > > > solution?
> > > > > >   
> > > > > 
> > > > > Currently, we only have VM stop on errors, that looks the same as a
> > > > > sudden power down to me.  With this solution, we have about
> > > > > 50%(non-fatal) chance to reduce the sudden power-down risk.
> > > > 
> > > > If half of all faults are expected to be non-fatal, then you must have
> > > > some real examples of devices triggering non-fatal errors which can be
> > > > corrected in the guest driver that you can share to justify why it's a
> > > > good thing to enable this behavior.
> > > > 
> > > > > What if a guest doesn't support AER?  It looks the same as a host
> > > > > without AER support. Now I only can speculate the worst condition: 
> > > > > guest
> > > > > crash, would that be quite different from a sudden power-down?
> > > > 
> > > > Yes, it's very different.  In one case we contain the fault by stopping
> > > > the guest, in the other case we allow the guest to continue operating
> > > > with a known fault in the device which may allow the fault to propagate
> > > > and perhaps go unnoticed.  We have established with the current
> > > > behavior that QEMU will prevent further propagation of a fault by
> > > > halting the VM.  To change QEMU's behavior here risks that a VM relying
> > > > on that behavior no longer has that protection.  So it seems we either
> > > > need to detect whether the VM is handling AER or we need to require the
> > > > VM administrator to opt-in to this new feature.
> > > 
> > > An opt-in flag sounds very reasonable. It can also specify whether
> > > to log the errors. We have a similar flag for disk errors.  
> > 
> > An opt-in works, but is rather burdensome to the user.
> >
> > > >  Real hardware has
> > > > these same issues and I believe there are handshakes that can be done
> > > > through ACPI to allow the guest to take over error handling from the
> > > > system.
> > > 
> > > No, that's only for error reporting IIUC. Driver needs to be
> > > aware of a chance for errors to trigger and be able to
> > > handle them.  
> > 
> > See drivers/acpi/pci_root.c:negotiate_os_control(), it seems that the
> > OSPM uses an _OSC to tell ACPI via OSC_PCI_EXPRESS_AER_CONTROL.  Would
> > that not be a reasonable mechanism for the guest to indicate AER
> > support?  
> 
> I'm not sure - it seems to be designed for firmware that can drive
> AER natively. E.g. if we ever set FIRMWARE_FIRST then linux
> will not set this bit.
> 
> It's also global so doesn't really indicate a given driver
> supports AER.
> 
> Still - would this remove some or all of your concern?

Certainly not all, I have rather deep concerns about where we're going
here.

> Could you outline a set of requirements that can be satisfied
> to make you consider the feature for inclusion?

Like any enhancement, show me that it's useful, show me that we've done
due diligence in researching the problem and solution, show me that
we're not painting ourselves into a corner by only addressing a subset
of the problem, show me that it's been thoroughly tested and reviewed.
Currently, I have little confidence in any of this.  We seem to just be
tossing AER spitballs at the wall hoping one of them sticks.
 
> I tried by writing up
>   vfio/pci: guest error recovery proposal
> back in December and there didn't seem to be any objections, so I am
> quite surprised to see patches implementing that proposal more or less
> verbatim getting rejected.

Are the concerns from the patch review not valid?  I think this is
indicative of the problems we've had throughout this 2+ year
development process, a suggestion is made and it's implemented without
a thorough 

Re: [Qemu-devel] [PATCH v3 3/3] vfio-pci: process non fatal error of AER

[Michael S. Tsirkin]

On Tue, Mar 28, 2017 at 08:55:04PM -0600, Alex Williamson wrote:
> On Wed, 29 Mar 2017 02:59:34 +0300
> "Michael S. Tsirkin"  wrote:
> 
> > On Tue, Mar 28, 2017 at 10:12:25AM -0600, Alex Williamson wrote:
> > > On Tue, 28 Mar 2017 21:49:17 +0800
> > > Cao jin  wrote:
> > >   
> > > > On 03/25/2017 06:12 AM, Alex Williamson wrote:  
> > > > > On Thu, 23 Mar 2017 17:09:23 +0800
> > > > > Cao jin  wrote:
> > > > > 
> > > > >> Make use of the non fatal error eventfd that the kernel module 
> > > > >> provide
> > > > >> to process the AER non fatal error. Fatal error still goes into the
> > > > >> legacy way which results in VM stop.
> > > > >>
> > > > >> Register the handler, wait for notification. Construct aer message 
> > > > >> and
> > > > >> pass it to root port on notification. Root port will trigger an 
> > > > >> interrupt
> > > > >> to signal guest, then guest driver will do the recovery.
> > > > > 
> > > > > Can we guarantee this is the better solution in all cases or could
> > > > > there be guests without AER support where the VM stop is the better
> > > > > solution?
> > > > > 
> > > > 
> > > > Currently, we only have VM stop on errors, that looks the same as a
> > > > sudden power down to me.  With this solution, we have about
> > > > 50%(non-fatal) chance to reduce the sudden power-down risk.  
> > > 
> > > If half of all faults are expected to be non-fatal, then you must have
> > > some real examples of devices triggering non-fatal errors which can be
> > > corrected in the guest driver that you can share to justify why it's a
> > > good thing to enable this behavior.
> > >   
> > > > What if a guest doesn't support AER?  It looks the same as a host
> > > > without AER support. Now I only can speculate the worst condition: guest
> > > > crash, would that be quite different from a sudden power-down?  
> > > 
> > > Yes, it's very different.  In one case we contain the fault by stopping
> > > the guest, in the other case we allow the guest to continue operating
> > > with a known fault in the device which may allow the fault to propagate
> > > and perhaps go unnoticed.  We have established with the current
> > > behavior that QEMU will prevent further propagation of a fault by
> > > halting the VM.  To change QEMU's behavior here risks that a VM relying
> > > on that behavior no longer has that protection.  So it seems we either
> > > need to detect whether the VM is handling AER or we need to require the
> > > VM administrator to opt-in to this new feature.  
> > 
> > An opt-in flag sounds very reasonable. It can also specify whether
> > to log the errors. We have a similar flag for disk errors.
> 
> An opt-in works, but is rather burdensome to the user.
>  
> > >  Real hardware has
> > > these same issues and I believe there are handshakes that can be done
> > > through ACPI to allow the guest to take over error handling from the
> > > system.  
> > 
> > No, that's only for error reporting IIUC. Driver needs to be
> > aware of a chance for errors to trigger and be able to
> > handle them.
> 
> See drivers/acpi/pci_root.c:negotiate_os_control(), it seems that the
> OSPM uses an _OSC to tell ACPI via OSC_PCI_EXPRESS_AER_CONTROL.  Would
> that not be a reasonable mechanism for the guest to indicate AER
> support?

I'm not sure - it seems to be designed for firmware that can drive
AER natively. E.g. if we ever set FIRMWARE_FIRST then linux
will not set this bit.

It's also global so doesn't really indicate a given driver
supports AER.

Still - would this remove some or all of your concern?

Could you outline a set of requirements that can be satisfied
to make you consider the feature for inclusion?

I tried by writing up
vfio/pci: guest error recovery proposal
back in December and there didn't seem to be any objections, so I am
quite surprised to see patches implementing that proposal more or less
verbatim getting rejected.

Could you write up a proposal of your own? It shouldn't take
long but I don't believe progress can be made otherwise.

Please note that at least in Linux most driver developers
test using software error injection. Documentation/PCI/pcieaer-howto.txt
actually says:

4. Software error injection

Debugging PCIe AER error recovery code is quite difficult because it
is hard to trigger real hardware errors. Software based error
injection can be used to fake various kinds of PCIe errors.

it might be that even though yes, we'd prefer real testing
we just might have to be satisfied with software injection.


> > So yes, some guests might have benefitted from VM stop
> > on AER but
> > 1. the stop happens asynchronously so if guest can't handle
> >errors there's a chance it is already crashed by the time we
> >try to do vm stop
> 
> I fully concede that it's asynchronous, bad data can propagate and a
> guest crash is one potential outcome.  That's fine, a 

Re: [Qemu-devel] [PATCH v3 3/3] vfio-pci: process non fatal error of AER

[Alex Williamson]

On Wed, 29 Mar 2017 02:59:34 +0300
"Michael S. Tsirkin"  wrote:

> On Tue, Mar 28, 2017 at 10:12:25AM -0600, Alex Williamson wrote:
> > On Tue, 28 Mar 2017 21:49:17 +0800
> > Cao jin  wrote:
> >   
> > > On 03/25/2017 06:12 AM, Alex Williamson wrote:  
> > > > On Thu, 23 Mar 2017 17:09:23 +0800
> > > > Cao jin  wrote:
> > > > 
> > > >> Make use of the non fatal error eventfd that the kernel module provide
> > > >> to process the AER non fatal error. Fatal error still goes into the
> > > >> legacy way which results in VM stop.
> > > >>
> > > >> Register the handler, wait for notification. Construct aer message and
> > > >> pass it to root port on notification. Root port will trigger an 
> > > >> interrupt
> > > >> to signal guest, then guest driver will do the recovery.
> > > > 
> > > > Can we guarantee this is the better solution in all cases or could
> > > > there be guests without AER support where the VM stop is the better
> > > > solution?
> > > > 
> > > 
> > > Currently, we only have VM stop on errors, that looks the same as a
> > > sudden power down to me.  With this solution, we have about
> > > 50%(non-fatal) chance to reduce the sudden power-down risk.  
> > 
> > If half of all faults are expected to be non-fatal, then you must have
> > some real examples of devices triggering non-fatal errors which can be
> > corrected in the guest driver that you can share to justify why it's a
> > good thing to enable this behavior.
> >   
> > > What if a guest doesn't support AER?  It looks the same as a host
> > > without AER support. Now I only can speculate the worst condition: guest
> > > crash, would that be quite different from a sudden power-down?  
> > 
> > Yes, it's very different.  In one case we contain the fault by stopping
> > the guest, in the other case we allow the guest to continue operating
> > with a known fault in the device which may allow the fault to propagate
> > and perhaps go unnoticed.  We have established with the current
> > behavior that QEMU will prevent further propagation of a fault by
> > halting the VM.  To change QEMU's behavior here risks that a VM relying
> > on that behavior no longer has that protection.  So it seems we either
> > need to detect whether the VM is handling AER or we need to require the
> > VM administrator to opt-in to this new feature.  
> 
> An opt-in flag sounds very reasonable. It can also specify whether
> to log the errors. We have a similar flag for disk errors.

An opt-in works, but is rather burdensome to the user.
 
> >  Real hardware has
> > these same issues and I believe there are handshakes that can be done
> > through ACPI to allow the guest to take over error handling from the
> > system.  
> 
> No, that's only for error reporting IIUC. Driver needs to be
> aware of a chance for errors to trigger and be able to
> handle them.

See drivers/acpi/pci_root.c:negotiate_os_control(), it seems that the
OSPM uses an _OSC to tell ACPI via OSC_PCI_EXPRESS_AER_CONTROL.  Would
that not be a reasonable mechanism for the guest to indicate AER
support?

> So yes, some guests might have benefitted from VM stop
> on AER but
> 1. the stop happens asynchronously so if guest can't handle
>errors there's a chance it is already crashed by the time we
>try to do vm stop

I fully concede that it's asynchronous, bad data can propagate and a
guest crash is one potential outcome.  That's fine, a guest crash
indicates a problem.  A VM stop also indicates a problem.  Potential
lack of a crash or VM stop is the worrisome case.

> 2. it's more of a chance by-product - we never promised
>guests that VMs would be more robust than bare metal

Does that make it not a regression if we change the behavior?  I
wouldn't exactly call it a chance by-product, perhaps it wasn't the
primary motivation, but it was considered.  Thanks,

Alex

> > > >> Signed-off-by: Dou Liyang 
> > > >> Signed-off-by: Cao jin 
> > > >> ---
> > > >>  hw/vfio/pci.c  | 202 
> > > >> +
> > > >>  hw/vfio/pci.h  |   2 +
> > > >>  linux-headers/linux/vfio.h |   2 +
> > > >>  3 files changed, 206 insertions(+)
> > > >>
> > > >> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> > > >> index 3d0d005..c6786d5 100644
> > > >> --- a/hw/vfio/pci.c
> > > >> +++ b/hw/vfio/pci.c
> > > >> @@ -2432,6 +2432,200 @@ static void vfio_put_device(VFIOPCIDevice 
> > > >> *vdev)
> > > >>  vfio_put_base_device(>vbasedev);
> > > >>  }
> > > >>  
> > > >> +static void vfio_non_fatal_err_notifier_handler(void *opaque)
> > > >> +{
> > > >> +VFIOPCIDevice *vdev = opaque;
> > > >> +PCIDevice *dev = >pdev;
> > > >> +PCIEAERMsg msg = {
> > > >> +.severity = PCI_ERR_ROOT_CMD_NONFATAL_EN,
> > > >> +.source_id = pci_requester_id(dev),
> > > >> +};
> > > >> +
> > > >> +if 
> > > >> 

Re: [Qemu-devel] [PATCH v3 3/3] vfio-pci: process non fatal error of AER

[Michael S. Tsirkin]

On Tue, Mar 28, 2017 at 10:12:25AM -0600, Alex Williamson wrote:
> On Tue, 28 Mar 2017 21:49:17 +0800
> Cao jin  wrote:
> 
> > On 03/25/2017 06:12 AM, Alex Williamson wrote:
> > > On Thu, 23 Mar 2017 17:09:23 +0800
> > > Cao jin  wrote:
> > >   
> > >> Make use of the non fatal error eventfd that the kernel module provide
> > >> to process the AER non fatal error. Fatal error still goes into the
> > >> legacy way which results in VM stop.
> > >>
> > >> Register the handler, wait for notification. Construct aer message and
> > >> pass it to root port on notification. Root port will trigger an interrupt
> > >> to signal guest, then guest driver will do the recovery.  
> > > 
> > > Can we guarantee this is the better solution in all cases or could
> > > there be guests without AER support where the VM stop is the better
> > > solution?
> > >   
> > 
> > Currently, we only have VM stop on errors, that looks the same as a
> > sudden power down to me.  With this solution, we have about
> > 50%(non-fatal) chance to reduce the sudden power-down risk.
> 
> If half of all faults are expected to be non-fatal, then you must have
> some real examples of devices triggering non-fatal errors which can be
> corrected in the guest driver that you can share to justify why it's a
> good thing to enable this behavior.
> 
> > What if a guest doesn't support AER?  It looks the same as a host
> > without AER support. Now I only can speculate the worst condition: guest
> > crash, would that be quite different from a sudden power-down?
> 
> Yes, it's very different.  In one case we contain the fault by stopping
> the guest, in the other case we allow the guest to continue operating
> with a known fault in the device which may allow the fault to propagate
> and perhaps go unnoticed.  We have established with the current
> behavior that QEMU will prevent further propagation of a fault by
> halting the VM.  To change QEMU's behavior here risks that a VM relying
> on that behavior no longer has that protection.  So it seems we either
> need to detect whether the VM is handling AER or we need to require the
> VM administrator to opt-in to this new feature.

An opt-in flag sounds very reasonable. It can also specify whether
to log the errors. We have a similar flag for disk errors.

>  Real hardware has
> these same issues and I believe there are handshakes that can be done
> through ACPI to allow the guest to take over error handling from the
> system.

No, that's only for error reporting IIUC. Driver needs to be
aware of a chance for errors to trigger and be able to
handle them.

So yes, some guests might have benefitted from VM stop
on AER but
1. the stop happens asynchronously so if guest can't handle
   errors there's a chance it is already crashed by the time we
   try to do vm stop
2. it's more of a chance by-product - we never promised
   guests that VMs would be more robust than bare metal



> > >> Signed-off-by: Dou Liyang 
> > >> Signed-off-by: Cao jin 
> > >> ---
> > >>  hw/vfio/pci.c  | 202 
> > >> +
> > >>  hw/vfio/pci.h  |   2 +
> > >>  linux-headers/linux/vfio.h |   2 +
> > >>  3 files changed, 206 insertions(+)
> > >>
> > >> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> > >> index 3d0d005..c6786d5 100644
> > >> --- a/hw/vfio/pci.c
> > >> +++ b/hw/vfio/pci.c
> > >> @@ -2432,6 +2432,200 @@ static void vfio_put_device(VFIOPCIDevice *vdev)
> > >>  vfio_put_base_device(>vbasedev);
> > >>  }
> > >>  
> > >> +static void vfio_non_fatal_err_notifier_handler(void *opaque)
> > >> +{
> > >> +VFIOPCIDevice *vdev = opaque;
> > >> +PCIDevice *dev = >pdev;
> > >> +PCIEAERMsg msg = {
> > >> +.severity = PCI_ERR_ROOT_CMD_NONFATAL_EN,
> > >> +.source_id = pci_requester_id(dev),
> > >> +};
> > >> +
> > >> +if (!event_notifier_test_and_clear(>non_fatal_err_notifier)) {
> > >> +return;
> > >> +}
> > >> +
> > >> +/* Populate the aer msg and send it to root port */
> > >> +if (dev->exp.aer_cap) {  
> > > 
> > > Why would we have registered this notifier otherwise?
> > >   
> > >> +uint8_t *aer_cap = dev->config + dev->exp.aer_cap;
> > >> +uint32_t uncor_status;
> > >> +bool isfatal;
> > >> +
> > >> +uncor_status = vfio_pci_read_config(dev,
> > >> +dev->exp.aer_cap + PCI_ERR_UNCOR_STATUS, 4);
> > >> +if (!uncor_status) {
> > >> +return;
> > >> +}
> > >> +
> > >> +isfatal = uncor_status & pci_get_long(aer_cap + 
> > >> PCI_ERR_UNCOR_SEVER);
> > >> +if (isfatal) {
> > >> +goto stop;
> > >> +}  
> > > 
> > > Huh?  How can we get a non-fatal error notice for a fatal error?  (and
> > > why are we saving this to a variable rather than testing it within the
> > > 'if' condition?
> > >  
> > 
> > Both 

Re: [Qemu-devel] [PATCH v3 3/3] vfio-pci: process non fatal error of AER

[Alex Williamson]

On Tue, 28 Mar 2017 21:49:17 +0800
Cao jin  wrote:

> On 03/25/2017 06:12 AM, Alex Williamson wrote:
> > On Thu, 23 Mar 2017 17:09:23 +0800
> > Cao jin  wrote:
> >   
> >> Make use of the non fatal error eventfd that the kernel module provide
> >> to process the AER non fatal error. Fatal error still goes into the
> >> legacy way which results in VM stop.
> >>
> >> Register the handler, wait for notification. Construct aer message and
> >> pass it to root port on notification. Root port will trigger an interrupt
> >> to signal guest, then guest driver will do the recovery.  
> > 
> > Can we guarantee this is the better solution in all cases or could
> > there be guests without AER support where the VM stop is the better
> > solution?
> >   
> 
> Currently, we only have VM stop on errors, that looks the same as a
> sudden power down to me.  With this solution, we have about
> 50%(non-fatal) chance to reduce the sudden power-down risk.

If half of all faults are expected to be non-fatal, then you must have
some real examples of devices triggering non-fatal errors which can be
corrected in the guest driver that you can share to justify why it's a
good thing to enable this behavior.

> What if a guest doesn't support AER?  It looks the same as a host
> without AER support. Now I only can speculate the worst condition: guest
> crash, would that be quite different from a sudden power-down?

Yes, it's very different.  In one case we contain the fault by stopping
the guest, in the other case we allow the guest to continue operating
with a known fault in the device which may allow the fault to propagate
and perhaps go unnoticed.  We have established with the current
behavior that QEMU will prevent further propagation of a fault by
halting the VM.  To change QEMU's behavior here risks that a VM relying
on that behavior no longer has that protection.  So it seems we either
need to detect whether the VM is handling AER or we need to require the
VM administrator to opt-in to this new feature.  Real hardware has
these same issues and I believe there are handshakes that can be done
through ACPI to allow the guest to take over error handling from the
system.

> >> Signed-off-by: Dou Liyang 
> >> Signed-off-by: Cao jin 
> >> ---
> >>  hw/vfio/pci.c  | 202 
> >> +
> >>  hw/vfio/pci.h  |   2 +
> >>  linux-headers/linux/vfio.h |   2 +
> >>  3 files changed, 206 insertions(+)
> >>
> >> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> >> index 3d0d005..c6786d5 100644
> >> --- a/hw/vfio/pci.c
> >> +++ b/hw/vfio/pci.c
> >> @@ -2432,6 +2432,200 @@ static void vfio_put_device(VFIOPCIDevice *vdev)
> >>  vfio_put_base_device(>vbasedev);
> >>  }
> >>  
> >> +static void vfio_non_fatal_err_notifier_handler(void *opaque)
> >> +{
> >> +VFIOPCIDevice *vdev = opaque;
> >> +PCIDevice *dev = >pdev;
> >> +PCIEAERMsg msg = {
> >> +.severity = PCI_ERR_ROOT_CMD_NONFATAL_EN,
> >> +.source_id = pci_requester_id(dev),
> >> +};
> >> +
> >> +if (!event_notifier_test_and_clear(>non_fatal_err_notifier)) {
> >> +return;
> >> +}
> >> +
> >> +/* Populate the aer msg and send it to root port */
> >> +if (dev->exp.aer_cap) {  
> > 
> > Why would we have registered this notifier otherwise?
> >   
> >> +uint8_t *aer_cap = dev->config + dev->exp.aer_cap;
> >> +uint32_t uncor_status;
> >> +bool isfatal;
> >> +
> >> +uncor_status = vfio_pci_read_config(dev,
> >> +dev->exp.aer_cap + PCI_ERR_UNCOR_STATUS, 4);
> >> +if (!uncor_status) {
> >> +return;
> >> +}
> >> +
> >> +isfatal = uncor_status & pci_get_long(aer_cap + 
> >> PCI_ERR_UNCOR_SEVER);
> >> +if (isfatal) {
> >> +goto stop;
> >> +}  
> > 
> > Huh?  How can we get a non-fatal error notice for a fatal error?  (and
> > why are we saving this to a variable rather than testing it within the
> > 'if' condition?
> >  
> 
> Both of these are for the unsure corner cases.
> Is it possible that register reading shows a fatal error?
> Saving it into a variable just is personal taste: more neat.

Why are there unsure corner cases?  Shouldn't the kernel have done this
check if there was any doubt whether the error was fatal or not?
Signaling the user with a non-fatal trigger for a fatal error certainly
doesn't make me have much confidence in this code.

> >> +
> >> +error_report("%s sending non fatal event to root port. uncor 
> >> status = "
> >> + "0x%"PRIx32, vdev->vbasedev.name, uncor_status);
> >> +pcie_aer_msg(dev, );
> >> +return;
> >> +}
> >> +
> >> +stop:
> >> +/* Terminate the guest in case of fatal error */
> >> +error_report("%s: Device detected a fatal error. VM stopped",
> >> +  vdev->vbasedev.name);

Re: [Qemu-devel] [PATCH v3 3/3] vfio-pci: process non fatal error of AER

[Cao jin]

On 03/25/2017 06:12 AM, Alex Williamson wrote:
> On Thu, 23 Mar 2017 17:09:23 +0800
> Cao jin  wrote:
> 
>> Make use of the non fatal error eventfd that the kernel module provide
>> to process the AER non fatal error. Fatal error still goes into the
>> legacy way which results in VM stop.
>>
>> Register the handler, wait for notification. Construct aer message and
>> pass it to root port on notification. Root port will trigger an interrupt
>> to signal guest, then guest driver will do the recovery.
> 
> Can we guarantee this is the better solution in all cases or could
> there be guests without AER support where the VM stop is the better
> solution?
> 

Currently, we only have VM stop on errors, that looks the same as a
sudden power down to me.  With this solution, we have about
50%(non-fatal) chance to reduce the sudden power-down risk.

What if a guest doesn't support AER?  It looks the same as a host
without AER support. Now I only can speculate the worst condition: guest
crash, would that be quite different from a sudden power-down?

>>
>> Signed-off-by: Dou Liyang 
>> Signed-off-by: Cao jin 
>> ---
>>  hw/vfio/pci.c  | 202 
>> +
>>  hw/vfio/pci.h  |   2 +
>>  linux-headers/linux/vfio.h |   2 +
>>  3 files changed, 206 insertions(+)
>>
>> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
>> index 3d0d005..c6786d5 100644
>> --- a/hw/vfio/pci.c
>> +++ b/hw/vfio/pci.c
>> @@ -2432,6 +2432,200 @@ static void vfio_put_device(VFIOPCIDevice *vdev)
>>  vfio_put_base_device(>vbasedev);
>>  }
>>  
>> +static void vfio_non_fatal_err_notifier_handler(void *opaque)
>> +{
>> +VFIOPCIDevice *vdev = opaque;
>> +PCIDevice *dev = >pdev;
>> +PCIEAERMsg msg = {
>> +.severity = PCI_ERR_ROOT_CMD_NONFATAL_EN,
>> +.source_id = pci_requester_id(dev),
>> +};
>> +
>> +if (!event_notifier_test_and_clear(>non_fatal_err_notifier)) {
>> +return;
>> +}
>> +
>> +/* Populate the aer msg and send it to root port */
>> +if (dev->exp.aer_cap) {
> 
> Why would we have registered this notifier otherwise?
> 
>> +uint8_t *aer_cap = dev->config + dev->exp.aer_cap;
>> +uint32_t uncor_status;
>> +bool isfatal;
>> +
>> +uncor_status = vfio_pci_read_config(dev,
>> +dev->exp.aer_cap + PCI_ERR_UNCOR_STATUS, 4);
>> +if (!uncor_status) {
>> +return;
>> +}
>> +
>> +isfatal = uncor_status & pci_get_long(aer_cap + 
>> PCI_ERR_UNCOR_SEVER);
>> +if (isfatal) {
>> +goto stop;
>> +}
> 
> Huh?  How can we get a non-fatal error notice for a fatal error?  (and
> why are we saving this to a variable rather than testing it within the
> 'if' condition?
>

Both of these are for the unsure corner cases.
Is it possible that register reading shows a fatal error?
Saving it into a variable just is personal taste: more neat.

>> +
>> +error_report("%s sending non fatal event to root port. uncor status 
>> = "
>> + "0x%"PRIx32, vdev->vbasedev.name, uncor_status);
>> +pcie_aer_msg(dev, );
>> +return;
>> +}
>> +
>> +stop:
>> +/* Terminate the guest in case of fatal error */
>> +error_report("%s: Device detected a fatal error. VM stopped",
>> +vdev->vbasedev.name);
>> +vm_stop(RUN_STATE_INTERNAL_ERROR);
> 
> Shouldn't we use the existing error index if we can't make use of
> correctable errors?
> 

Why? If register reading shows it is actually a fatal error, is it the
same as fatal error handler is notified?  what we use the existing error
index for?


>> @@ -2860,6 +3054,8 @@ static void vfio_realize(PCIDevice *pdev, Error **errp)
>>  }
>>  }
>>  
>> +vfio_register_passive_reset_notifier(vdev);
>> +vfio_register_non_fatal_err_notifier(vdev);
> 
> I think it's wrong that we configure these unconditionally.  Why do we
> care about these unless we're configuring the guest to receive AER
> events?
> 

But do we have ways to know whether the guest has AER support? For now,
I don't think so.

If guest don't have AER support, for the worst condition: guest crash,
it is not worse than a sudden power-down.


-- 
Sincerely,
Cao jin

Re: [Qemu-devel] [PATCH v3 3/3] vfio-pci: process non fatal error of AER

[Alex Williamson]

On Thu, 23 Mar 2017 17:09:23 +0800
Cao jin  wrote:

> Make use of the non fatal error eventfd that the kernel module provide
> to process the AER non fatal error. Fatal error still goes into the
> legacy way which results in VM stop.
> 
> Register the handler, wait for notification. Construct aer message and
> pass it to root port on notification. Root port will trigger an interrupt
> to signal guest, then guest driver will do the recovery.

Can we guarantee this is the better solution in all cases or could
there be guests without AER support where the VM stop is the better
solution?

> 
> Signed-off-by: Dou Liyang 
> Signed-off-by: Cao jin 
> ---
>  hw/vfio/pci.c  | 202 
> +
>  hw/vfio/pci.h  |   2 +
>  linux-headers/linux/vfio.h |   2 +
>  3 files changed, 206 insertions(+)
> 
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index 3d0d005..c6786d5 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -2432,6 +2432,200 @@ static void vfio_put_device(VFIOPCIDevice *vdev)
>  vfio_put_base_device(>vbasedev);
>  }
>  
> +static void vfio_non_fatal_err_notifier_handler(void *opaque)
> +{
> +VFIOPCIDevice *vdev = opaque;
> +PCIDevice *dev = >pdev;
> +PCIEAERMsg msg = {
> +.severity = PCI_ERR_ROOT_CMD_NONFATAL_EN,
> +.source_id = pci_requester_id(dev),
> +};
> +
> +if (!event_notifier_test_and_clear(>non_fatal_err_notifier)) {
> +return;
> +}
> +
> +/* Populate the aer msg and send it to root port */
> +if (dev->exp.aer_cap) {

Why would we have registered this notifier otherwise?

> +uint8_t *aer_cap = dev->config + dev->exp.aer_cap;
> +uint32_t uncor_status;
> +bool isfatal;
> +
> +uncor_status = vfio_pci_read_config(dev,
> +dev->exp.aer_cap + PCI_ERR_UNCOR_STATUS, 4);
> +if (!uncor_status) {
> +return;
> +}
> +
> +isfatal = uncor_status & pci_get_long(aer_cap + PCI_ERR_UNCOR_SEVER);
> +if (isfatal) {
> +goto stop;
> +}

Huh?  How can we get a non-fatal error notice for a fatal error?  (and
why are we saving this to a variable rather than testing it within the
'if' condition?

> +
> +error_report("%s sending non fatal event to root port. uncor status 
> = "
> + "0x%"PRIx32, vdev->vbasedev.name, uncor_status);
> +pcie_aer_msg(dev, );
> +return;
> +}
> +
> +stop:
> +/* Terminate the guest in case of fatal error */
> +error_report("%s: Device detected a fatal error. VM stopped",
> + vdev->vbasedev.name);
> +vm_stop(RUN_STATE_INTERNAL_ERROR);

Shouldn't we use the existing error index if we can't make use of
correctable errors?

> +}
> +
> +/*
> + * Register non fatal error notifier for devices supporting error recovery.
> + * If we encounter a failure in this function, we report an error
> + * and continue after disabling error recovery support for the device.
> + */
> +static void vfio_register_non_fatal_err_notifier(VFIOPCIDevice *vdev)
> +{
> +int ret;
> +int argsz;
> +struct vfio_irq_set *irq_set;
> +int32_t *pfd;
> +
> +if (event_notifier_init(>non_fatal_err_notifier, 0)) {
> +error_report("vfio: Unable to init event notifier for non-fatal 
> error detection");
> +return;
> +}
> +
> +argsz = sizeof(*irq_set) + sizeof(*pfd);
> +
> +irq_set = g_malloc0(argsz);
> +irq_set->argsz = argsz;
> +irq_set->flags = VFIO_IRQ_SET_DATA_EVENTFD |
> + VFIO_IRQ_SET_ACTION_TRIGGER;
> +irq_set->index = VFIO_PCI_NON_FATAL_ERR_IRQ_INDEX;
> +irq_set->start = 0;
> +irq_set->count = 1;
> +pfd = (int32_t *)_set->data;
> +
> +*pfd = event_notifier_get_fd(>non_fatal_err_notifier);
> +qemu_set_fd_handler(*pfd, vfio_non_fatal_err_notifier_handler, NULL, 
> vdev);
> +
> +ret = ioctl(vdev->vbasedev.fd, VFIO_DEVICE_SET_IRQS, irq_set);
> +if (ret) {
> +error_report("vfio: Failed to set up non-fatal error notification: 
> %m");
> +qemu_set_fd_handler(*pfd, NULL, NULL, vdev);
> +event_notifier_cleanup(>non_fatal_err_notifier);
> +}
> +g_free(irq_set);
> +}
> +
> +static void vfio_unregister_non_fatal_err_notifier(VFIOPCIDevice *vdev)
> +{
> +int argsz;
> +struct vfio_irq_set *irq_set;
> +int32_t *pfd;
> +int ret;
> +
> +argsz = sizeof(*irq_set) + sizeof(*pfd);
> +
> +irq_set = g_malloc0(argsz);
> +irq_set->argsz = argsz;
> +irq_set->flags = VFIO_IRQ_SET_DATA_EVENTFD |
> + VFIO_IRQ_SET_ACTION_TRIGGER;
> +irq_set->index = VFIO_PCI_NON_FATAL_ERR_IRQ_INDEX;
> +irq_set->start = 0;
> +irq_set->count = 1;
> +pfd = (int32_t *)_set->data;
> +*pfd = -1;
> +
> +ret = ioctl(vdev->vbasedev.fd, VFIO_DEVICE_SET_IRQS, irq_set);
> +if (ret) {
> +