Re: [PATCH v13 3/3] iommu/vt-d: improve ITE fault handling if target device isn't valid

Dan Carpenter
2024-02-23T06:08:58Z

On Fri, Feb 23, 2024 at 10:29:28AM +0800, Ethan Zhao wrote:
> > > @@ -1326,6 +1336,21 @@ static int qi_check_fault(struct intel_iommu *iommu, int index, int wait_index)
> > > head = (head - 2 + QI_LENGTH) % QI_LENGTH;
> > > } while (head != tail);
> > > + /*
> > > + * If got ITE, we need to check if the sid of ITE is one of the
> > > + * current valid ATS invalidation target devices, if no, or the
> > > + * target device isn't presnet, don't try this request anymore.
> > > + * 0 value of ite_sid means old VT-d device, no ite_sid value.
> > > + */
> > This comment is kind of confusing.
>
> Really confusing ? this is typo there, resnet-> "present"
>

Reading this comment again, the part about zero ite_sid values is
actually useful, but what does "old" mean in "old VT-d device". How old
is it? One year old?

> >
> > /*
> > * If we have an ITE, then we need to check whether the sid of the ITE
> > * is in the rbtree (meaning it is probed and not released), and that
> > * the PCI device is present.
> > */
> >
> > My comment is slightly shorter but I think it has the necessary
> > information.
> >
> > > + if (ite_sid) {
> > > + dev = device_rbtree_find(iommu, ite_sid);
> > > + if (!dev || !dev_is_pci(dev))
> > > + return -ETIMEDOUT;
> > -ETIMEDOUT is weird. The callers don't care which error code we return.
> > Change this to -ENODEV or something
>
> -ETIMEDOUT means prior ATS invalidation request hit timeout fault, and the
> caller really cares about the returned value.
>

I don't really care about the return value and if you say it should be
-ETIMEDOUT, then you're the expert. However, I don't see anything in
linux-next which cares about the return values except -EAGAIN.
This function is only called from qi_submit_sync() which checks for
-EAGAIN. Then I did a git grep.

$ git grep qi_submit_sync
drivers/iommu/intel/dmar.c:int qi_submit_sync(struct intel_iommu *iommu, struct qi_desc *desc,
drivers/iommu/intel/dmar.c: qi_submit_sync(iommu, &desc, 1, 0);
drivers/iommu/intel/dmar.c: qi_submit_sync(iommu, &desc, 1, 0);
drivers/iommu/intel/dmar.c: qi_submit_sync(iommu, &desc, 1, 0);
drivers/iommu/intel/dmar.c: qi_submit_sync(iommu, &desc, 1, 0);
drivers/iommu/intel/dmar.c: qi_submit_sync(iommu, &desc, 1, 0);
drivers/iommu/intel/dmar.c: qi_submit_sync(iommu, &desc, 1, 0);
drivers/iommu/intel/dmar.c: qi_submit_sync(iommu, &desc, 1, 0);
drivers/iommu/intel/iommu.h:int qi_submit_sync(struct intel_iommu *iommu, struct qi_desc *desc,
drivers/iommu/intel/iommu.h: * Options used in qi_submit_sync:
drivers/iommu/intel/irq_remapping.c: return qi_submit_sync(iommu, &desc, 1, 0);
drivers/iommu/intel/pasid.c: qi_submit_sync(iommu, &desc, 1, 0);
drivers/iommu/intel/svm.c: qi_submit_sync(iommu, desc, 3, QI_OPT_WAIT_DRAIN);
drivers/iommu/intel/svm.c: qi_submit_sync(iommu, &desc, 1, 0);
drivers/iommu/intel/svm.c: qi_submit_sync(iommu, &desc, 1, 0);

Only qi_flush_iec() in irq_remapping.c cares about the return. Then I
traced those callers back and nothing cares about -ETIMEOUT.

Are you refering to patches that haven't ben merged yet?

> >
> > > + pdev = to_pci_dev(dev);
> > > + if (!pci_device_is_present(pdev) &&
> > > + ite_sid == pci_dev_id(pci_physfn(pdev)))
> > The && confused me, but then I realized that probably "ite_sid ==
> > pci_dev_id(pci_physfn(pdev))" is always true. Can we delete that part?
>
> Here is the fault handling, just double confirm nothing else goes wrong --
> beyond the assumption.
>

Basically for that to ever be != it would need some kind of memory
corruption? I feel like in that situation, the more conservative thing
is to give up. If the PCI device is not present then just give up.

regards,
dan carpenter