Re: [PATCH v4 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group
On 2022/7/1 04:36, Nicolin Chen wrote: Cases like VFIO wish to attach a device to an existing domain that was not allocated specifically from the device. This raises a condition where the IOMMU driver can fail the domain attach because the domain and device are incompatible with each other. This is a soft failure that can be resolved by using a different domain. Provide a dedicated errno from the IOMMU driver during attach that the reason attached failed is because of domain incompatability. EMEDIUMTYPE is chosen because it is never used within the iommu subsystem today and evokes a sense that the 'medium' aka the domain is incompatible. VFIO can use this to know attach is a soft failure and it should continue searching. Otherwise the attach will be a hard failure and VFIO will return the code to userspace. Update all drivers to return EMEDIUMTYPE in their failure paths that are related to domain incompatability. Also remove adjacent error prints for these soft failures, to prevent a kernel log spam, since -EMEDIUMTYPE is clear enough to indicate an incompatability error. Add kdocs describing this behavior. Suggested-by: Jason Gunthorpe Reviewed-by: Kevin Tian Signed-off-by: Nicolin Chen Reviewed-by: Lu Baolu Best regards, baolu ___ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
Re: [PATCH v4 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group
On 2022-06-30 21:36, Nicolin Chen wrote:
Cases like VFIO wish to attach a device to an existing domain that was
not allocated specifically from the device. This raises a condition
where the IOMMU driver can fail the domain attach because the domain and
device are incompatible with each other.
This is a soft failure that can be resolved by using a different domain.
Provide a dedicated errno from the IOMMU driver during attach that the
reason attached failed is because of domain incompatability. EMEDIUMTYPE
is chosen because it is never used within the iommu subsystem today and
evokes a sense that the 'medium' aka the domain is incompatible.
VFIO can use this to know attach is a soft failure and it should continue
searching. Otherwise the attach will be a hard failure and VFIO will
return the code to userspace.
Update all drivers to return EMEDIUMTYPE in their failure paths that are
related to domain incompatability. Also remove adjacent error prints for
these soft failures, to prevent a kernel log spam, since -EMEDIUMTYPE is
clear enough to indicate an incompatability error.
Add kdocs describing this behavior.
Suggested-by: Jason Gunthorpe
Reviewed-by: Kevin Tian
Signed-off-by: Nicolin Chen
---
[...]
diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c
b/drivers/iommu/arm/arm-smmu/arm-smmu.c
index 2ed3594f384e..072cac5ab5a4 100644
--- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
+++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
@@ -1135,10 +1135,8 @@ static int arm_smmu_attach_dev(struct iommu_domain
*domain, struct device *dev)
struct arm_smmu_device *smmu;
int ret;
- if (!fwspec || fwspec->ops != &arm_smmu_ops) {
- dev_err(dev, "cannot attach to SMMU, is it on the same bus?\n");
- return -ENXIO;
- }
+ if (!fwspec || fwspec->ops != &arm_smmu_ops)
+ return -EMEDIUMTYPE;
This is the wrong check, you want the "if (smmu_domain->smmu != smmu)"
condition further down. If this one fails it's effectively because the
device doesn't have an IOMMU at all, and similar to patch #3 it will be
removed once the core code takes over properly (I even have both those
patches written now!)
Thanks,
Robin.
/*
* FIXME: The arch/arm DMA API code tries to attach devices to its own
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Re: [PATCH v4 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group
On Fri, Jul 01, 2022 at 11:21:48AM +0100, Robin Murphy wrote:
> > diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c
> > b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> > index 2ed3594f384e..072cac5ab5a4 100644
> > --- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
> > +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> > @@ -1135,10 +1135,8 @@ static int arm_smmu_attach_dev(struct iommu_domain
> > *domain, struct device *dev)
> > struct arm_smmu_device *smmu;
> > int ret;
> >
> > - if (!fwspec || fwspec->ops != &arm_smmu_ops) {
> > - dev_err(dev, "cannot attach to SMMU, is it on the same
> > bus?\n");
> > - return -ENXIO;
> > - }
> > + if (!fwspec || fwspec->ops != &arm_smmu_ops)
> > + return -EMEDIUMTYPE;
>
> This is the wrong check, you want the "if (smmu_domain->smmu != smmu)"
> condition further down. If this one fails it's effectively because the
> device doesn't have an IOMMU at all, and similar to patch #3 it will be
Thanks for the review! I will fix that. The "on the same bus" is
quite eye-catching.
> removed once the core code takes over properly (I even have both those
> patches written now!)
Actually in my v1 the proposal for ops check returned -EMEDIUMTYPE
also upon an ops mismatch, treating that too as an incompatibility.
Do you mean that we should have fine-grained it further?
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Re: [PATCH v4 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group
On 01/07/2022 5:43 pm, Nicolin Chen wrote:
On Fri, Jul 01, 2022 at 11:21:48AM +0100, Robin Murphy wrote:
diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c
b/drivers/iommu/arm/arm-smmu/arm-smmu.c
index 2ed3594f384e..072cac5ab5a4 100644
--- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
+++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
@@ -1135,10 +1135,8 @@ static int arm_smmu_attach_dev(struct iommu_domain
*domain, struct device *dev)
struct arm_smmu_device *smmu;
int ret;
- if (!fwspec || fwspec->ops != &arm_smmu_ops) {
- dev_err(dev, "cannot attach to SMMU, is it on the same bus?\n");
- return -ENXIO;
- }
+ if (!fwspec || fwspec->ops != &arm_smmu_ops)
+ return -EMEDIUMTYPE;
This is the wrong check, you want the "if (smmu_domain->smmu != smmu)"
condition further down. If this one fails it's effectively because the
device doesn't have an IOMMU at all, and similar to patch #3 it will be
Thanks for the review! I will fix that. The "on the same bus" is
quite eye-catching.
removed once the core code takes over properly (I even have both those
patches written now!)
Actually in my v1 the proposal for ops check returned -EMEDIUMTYPE
also upon an ops mismatch, treating that too as an incompatibility.
Do you mean that we should have fine-grained it further?
On second look, I think this particular check was already entirely
redundant by the time I made the fwspec conversion to it, oh well. Since
it remains harmless for the time being, let's just ignore it entirely
until we can confidently say goodbye to the whole lot[1].
I don't think there's any need to differentiate an instance mismatch
from a driver mismatch, once the latter becomes realistically possible,
mostly due to iommu_domain_alloc() also having to become device-aware to
know which driver to allocate from. Thus as far as a user is concerned,
if attaching a device to an existing domain fails with -EMEDIUMTYPE,
allocating a new domain using the given device, and attaching to that,
can be expected to succeed, regardless of why the original attempt was
rejected. In fact even in the theoretical different-driver-per-bus model
the same principle still holds up.
Thanks,
Robin.
[1]
https://gitlab.arm.com/linux-arm/linux-rm/-/commit/aa4accfa4a10e92daad0d51095918e8a89014393
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Re: [PATCH v4 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group
On Fri, Jul 01, 2022 at 07:17:38PM +0100, Robin Murphy wrote:
> External email: Use caution opening links or attachments
>
>
> On 01/07/2022 5:43 pm, Nicolin Chen wrote:
> > On Fri, Jul 01, 2022 at 11:21:48AM +0100, Robin Murphy wrote:
> >
> > > > diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c
> > > > b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> > > > index 2ed3594f384e..072cac5ab5a4 100644
> > > > --- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
> > > > +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> > > > @@ -1135,10 +1135,8 @@ static int arm_smmu_attach_dev(struct
> > > > iommu_domain *domain, struct device *dev)
> > > >struct arm_smmu_device *smmu;
> > > >int ret;
> > > >
> > > > - if (!fwspec || fwspec->ops != &arm_smmu_ops) {
> > > > - dev_err(dev, "cannot attach to SMMU, is it on the same
> > > > bus?\n");
> > > > - return -ENXIO;
> > > > - }
> > > > + if (!fwspec || fwspec->ops != &arm_smmu_ops)
> > > > + return -EMEDIUMTYPE;
> > >
> > > This is the wrong check, you want the "if (smmu_domain->smmu != smmu)"
> > > condition further down. If this one fails it's effectively because the
> > > device doesn't have an IOMMU at all, and similar to patch #3 it will be
> >
> > Thanks for the review! I will fix that. The "on the same bus" is
> > quite eye-catching.
> >
> > > removed once the core code takes over properly (I even have both those
> > > patches written now!)
> >
> > Actually in my v1 the proposal for ops check returned -EMEDIUMTYPE
> > also upon an ops mismatch, treating that too as an incompatibility.
> > Do you mean that we should have fine-grained it further?
>
> On second look, I think this particular check was already entirely
> redundant by the time I made the fwspec conversion to it, oh well. Since
> it remains harmless for the time being, let's just ignore it entirely
> until we can confidently say goodbye to the whole lot[1].
That looks cleaner!
> I don't think there's any need to differentiate an instance mismatch
> from a driver mismatch, once the latter becomes realistically possible,
> mostly due to iommu_domain_alloc() also having to become device-aware to
> know which driver to allocate from. Thus as far as a user is concerned,
> if attaching a device to an existing domain fails with -EMEDIUMTYPE,
> allocating a new domain using the given device, and attaching to that,
> can be expected to succeed, regardless of why the original attempt was
> rejected. In fact even in the theoretical different-driver-per-bus model
> the same principle still holds up.
I see. Thanks for the explanation.
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