Re: [Qemu-devel] [PATCH] vfio: fix duplicate function call
On 10/17/2016 11:01 PM, Alex Williamson wrote:
On Mon, 17 Oct 2016 16:57:08 +0800
Cao jin wrote:
Hi,
On 10/14/2016 11:50 PM, Alex Williamson wrote:
On Fri, 14 Oct 2016 19:16:59 +0800
Cao jin wrote:
When vfio device is reset(encounter FLR, or bus reset), if need to do
bus reset(vfio_pci_hot_reset_one is called), vfio_pci_pre_reset &
vfio_pci_post_reset will be called twice.
Signed-off-by: Cao jin
---
Also has a little question on vfio_pci_reset. it will be called when encounter
bus reset, or FLR. The reset method's priority in this function now is:
1. If has "device specific reset function", then do it
2. If has FLR, then do it.
3. If it can do bus reset(only 1 affected device), then do it
4. If has pm_reset, then do it
The question is: why pm reset has low priority than bus reset(if it does
can do a bus reset)? why bus reset is not the last choice? In PCI driver
of kernel, pls see __pci_dev_reset, we can see, if device support pm reset,
it won't do bus reset.
The PCI spec doesn't really define what sort of reset is done with a PM
reset. My thinking was that if a device advertises an FLR capability
then the hardware has made a concerted effort to have a per function
reset mechanism available. NoSoftRst- is not terribly common and it's
not entirely clear to me that the hardware has made a conscious effort
to provide this for the purposes of per function reset mechanism.
Therefore I've opt'd to prioritize a bus reset over a PM reset.
I still have a question about vfio_pci_reset. I checked commit message
in f16f39c3, if I understand right, couldn't we put
/* See if we can do our own bus reset */
if (!vfio_pci_hot_reset_one(vdev)) {
goto post_reset;
}
in the 1st priority? Because if there is 1 affected device, then it will
do bus reset which is the best reset we can do; if there are more than 1
affected devices, after this patch, vfio_pci_hot_reset_one will do
nothing, and then try other reset methods.
It's possible, yes, but that disregards that the hardware has gone to
the trouble to implement a proper function level reset. As I
explained, I de-prioritize PM reset, specifically because I'm not sure
if hardware designers are necessarily intending it for the purpose of a
device reset. For FLR this is the entire purpose of the interface. We
also have a fair bit of experience with the current priority scheme and
I would not take it lightly to change without some compelling evidence
to prove that a new priority scheme is better than the existing. There
do also exist devices which do not behave properly with a secondary bus
reset, see drivers/pci/quirks.c:quirk_no_bus_reset() in the kernel
tree. It's possible more devices like this exist, but we don't see
them because they implement FLR. A bus reset may result in a more
complete device reset, but it's also more disruptive to the system.
Thanks,
Alex
I see. Thanks Alex, I think these are valuable info to me, although
maybe I still need more time in the future to understand these totally.
--
Yours Sincerely,
Cao jin
Re: [Qemu-devel] [PATCH] vfio: fix duplicate function call
On Mon, 17 Oct 2016 16:57:08 +0800
Cao jin wrote:
> Hi,
>
> On 10/14/2016 11:50 PM, Alex Williamson wrote:
> > On Fri, 14 Oct 2016 19:16:59 +0800
> > Cao jin wrote:
> >
> >> When vfio device is reset(encounter FLR, or bus reset), if need to do
> >> bus reset(vfio_pci_hot_reset_one is called), vfio_pci_pre_reset &
> >> vfio_pci_post_reset will be called twice.
> >>
> >> Signed-off-by: Cao jin
> >> ---
> >> Also has a little question on vfio_pci_reset. it will be called when
> >> encounter
> >> bus reset, or FLR. The reset method's priority in this function now is:
> >>
> >> 1. If has "device specific reset function", then do it
> >> 2. If has FLR, then do it.
> >> 3. If it can do bus reset(only 1 affected device), then do it
> >> 4. If has pm_reset, then do it
> >>
> >> The question is: why pm reset has low priority than bus reset(if it does
> >> can do a bus reset)? why bus reset is not the last choice? In PCI driver
> >> of kernel, pls see __pci_dev_reset, we can see, if device support pm reset,
> >> it won't do bus reset.
> >
> > The PCI spec doesn't really define what sort of reset is done with a PM
> > reset. My thinking was that if a device advertises an FLR capability
> > then the hardware has made a concerted effort to have a per function
> > reset mechanism available. NoSoftRst- is not terribly common and it's
> > not entirely clear to me that the hardware has made a conscious effort
> > to provide this for the purposes of per function reset mechanism.
> > Therefore I've opt'd to prioritize a bus reset over a PM reset.
> >
>
> I still have a question about vfio_pci_reset. I checked commit message
> in f16f39c3, if I understand right, couldn't we put
>
> /* See if we can do our own bus reset */
> if (!vfio_pci_hot_reset_one(vdev)) {
> goto post_reset;
> }
>
> in the 1st priority? Because if there is 1 affected device, then it will
> do bus reset which is the best reset we can do; if there are more than 1
> affected devices, after this patch, vfio_pci_hot_reset_one will do
> nothing, and then try other reset methods.
It's possible, yes, but that disregards that the hardware has gone to
the trouble to implement a proper function level reset. As I
explained, I de-prioritize PM reset, specifically because I'm not sure
if hardware designers are necessarily intending it for the purpose of a
device reset. For FLR this is the entire purpose of the interface. We
also have a fair bit of experience with the current priority scheme and
I would not take it lightly to change without some compelling evidence
to prove that a new priority scheme is better than the existing. There
do also exist devices which do not behave properly with a secondary bus
reset, see drivers/pci/quirks.c:quirk_no_bus_reset() in the kernel
tree. It's possible more devices like this exist, but we don't see
them because they implement FLR. A bus reset may result in a more
complete device reset, but it's also more disruptive to the system.
Thanks,
Alex
Re: [Qemu-devel] [PATCH] vfio: fix duplicate function call
Hi,
On 10/14/2016 11:50 PM, Alex Williamson wrote:
On Fri, 14 Oct 2016 19:16:59 +0800
Cao jin wrote:
When vfio device is reset(encounter FLR, or bus reset), if need to do
bus reset(vfio_pci_hot_reset_one is called), vfio_pci_pre_reset &
vfio_pci_post_reset will be called twice.
Signed-off-by: Cao jin
---
Also has a little question on vfio_pci_reset. it will be called when encounter
bus reset, or FLR. The reset method's priority in this function now is:
1. If has "device specific reset function", then do it
2. If has FLR, then do it.
3. If it can do bus reset(only 1 affected device), then do it
4. If has pm_reset, then do it
The question is: why pm reset has low priority than bus reset(if it does
can do a bus reset)? why bus reset is not the last choice? In PCI driver
of kernel, pls see __pci_dev_reset, we can see, if device support pm reset,
it won't do bus reset.
The PCI spec doesn't really define what sort of reset is done with a PM
reset. My thinking was that if a device advertises an FLR capability
then the hardware has made a concerted effort to have a per function
reset mechanism available. NoSoftRst- is not terribly common and it's
not entirely clear to me that the hardware has made a conscious effort
to provide this for the purposes of per function reset mechanism.
Therefore I've opt'd to prioritize a bus reset over a PM reset.
I still have a question about vfio_pci_reset. I checked commit message
in f16f39c3, if I understand right, couldn't we put
/* See if we can do our own bus reset */
if (!vfio_pci_hot_reset_one(vdev)) {
goto post_reset;
}
in the 1st priority? Because if there is 1 affected device, then it will
do bus reset which is the best reset we can do; if there are more than 1
affected devices, after this patch, vfio_pci_hot_reset_one will do
nothing, and then try other reset methods.
--
Yours Sincerely,
Cao jin
Re: [Qemu-devel] [PATCH] vfio: fix duplicate function call
On 10/14/2016 11:50 PM, Alex Williamson wrote: On Fri, 14 Oct 2016 19:16:59 +0800 Cao jin wrote: When vfio device is reset(encounter FLR, or bus reset), if need to do bus reset(vfio_pci_hot_reset_one is called), vfio_pci_pre_reset & vfio_pci_post_reset will be called twice. Signed-off-by: Cao jin --- Also has a little question on vfio_pci_reset. it will be called when encounter bus reset, or FLR. The reset method's priority in this function now is: 1. If has "device specific reset function", then do it 2. If has FLR, then do it. 3. If it can do bus reset(only 1 affected device), then do it 4. If has pm_reset, then do it The question is: why pm reset has low priority than bus reset(if it does can do a bus reset)? why bus reset is not the last choice? In PCI driver of kernel, pls see __pci_dev_reset, we can see, if device support pm reset, it won't do bus reset. The PCI spec doesn't really define what sort of reset is done with a PM reset. My thinking was that if a device advertises an FLR capability then the hardware has made a concerted effort to have a per function reset mechanism available. NoSoftRst- is not terribly common and it's not entirely clear to me that the hardware has made a conscious effort to provide this for the purposes of per function reset mechanism. Therefore I've opt'd to prioritize a bus reset over a PM reset. I see, thanks for your infomation. -- Yours Sincerely, Cao jin
Re: [Qemu-devel] [PATCH] vfio: fix duplicate function call
On Fri, 14 Oct 2016 19:16:59 +0800
Cao jin wrote:
> When vfio device is reset(encounter FLR, or bus reset), if need to do
> bus reset(vfio_pci_hot_reset_one is called), vfio_pci_pre_reset &
> vfio_pci_post_reset will be called twice.
>
> Signed-off-by: Cao jin
> ---
> Also has a little question on vfio_pci_reset. it will be called when encounter
> bus reset, or FLR. The reset method's priority in this function now is:
>
> 1. If has "device specific reset function", then do it
> 2. If has FLR, then do it.
> 3. If it can do bus reset(only 1 affected device), then do it
> 4. If has pm_reset, then do it
>
> The question is: why pm reset has low priority than bus reset(if it does
> can do a bus reset)? why bus reset is not the last choice? In PCI driver
> of kernel, pls see __pci_dev_reset, we can see, if device support pm reset,
> it won't do bus reset.
The PCI spec doesn't really define what sort of reset is done with a PM
reset. My thinking was that if a device advertises an FLR capability
then the hardware has made a concerted effort to have a per function
reset mechanism available. NoSoftRst- is not terribly common and it's
not entirely clear to me that the hardware has made a conscious effort
to provide this for the purposes of per function reset mechanism.
Therefore I've opt'd to prioritize a bus reset over a PM reset.
> hw/vfio/pci.c | 8 ++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index cce3024..ca4d1c1 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -1930,7 +1930,9 @@ static int vfio_pci_hot_reset(VFIOPCIDevice *vdev, bool
> single)
>
> trace_vfio_pci_hot_reset(vdev->vbasedev.name, single ? "one" : "multi");
>
> -vfio_pci_pre_reset(vdev);
> +if (!single) {
> +vfio_pci_pre_reset(vdev);
> +}
> vdev->vbasedev.needs_reset = false;
>
> info = g_malloc0(sizeof(*info));
> @@ -2088,7 +2090,9 @@ out:
> }
> }
> out_single:
> -vfio_pci_post_reset(vdev);
> +if (!single) {
> +vfio_pci_post_reset(vdev);
> +}
> g_free(info);
>
> return ret;
Looks ok to me, I'll queue it. Thanks,
Alex
