On Fri, Feb 23, 2024 at 10:29:28AM +0800, Ethan Zhao wrote:
> On 2/22/2024 7:24 PM, Dan Carpenter wrote:
> > On Thu, Feb 22, 2024 at 04:02:51AM -0500, Ethan Zhao wrote:
> > > Because surprise removal could happen anytime, e.g. user could request safe
> > > removal to EP(endpoint device) via sysfs and brings its link down to do
> > > surprise removal cocurrently. such aggressive cases would cause ATS
> > > invalidation request issued to non-existence target device, then deadly
> > > loop to retry that request after ITE fault triggered in interrupt context.
> > > this patch aims to optimize the ITE handling by checking the target device
> > > presence state to avoid retrying the timeout request blindly, thus avoid
> > > hard lockup or system hang.
> > >
> > > Devices are valid ATS invalidation request target only when they reside
> > > in the iommu->device_rbtree (probed, not released) and present.
> >
> > "valid invalidation" is awkward wording. Can we instead say:
>
> If you read them together, sounds like tongue twister. but here "ATS
> invalidation request target" is one term in PCIe spec.
"ATS invalidation request target" does not appear in the PCIe spec. I
think you're trying to avoid sending ATS Invalidate Requests when you
know they will not be completed.
It is impossible to reliably determine whether a device will be
present and able to complete an Invalidate Request. No matter what
you check to determine that a device is present *now*, it may be
removed before an Invalidate Request reaches it.
If an Invalidate Request to a non-existent device causes a "deadly
loop" (I'm not sure what that means) or a hard lockup or a system
hang, something is wrong with the hardware. There should be a
mechanism to recover from a timeout in that situation.
You can avoid sending Invalidate Requests to devices that have been
removed, and that will reduce the number of timeout cases. But if you
rely on a check like pci_device_is_present() or
pci_dev_is_disconnected(), there is *always* an unavoidable race
between a device removal and the Invalidate Request.
> > > @@ -1273,6 +1273,9 @@ static int qi_check_fault(struct intel_iommu *iommu, int index, int wait_index)
> > > {
> > > u32 fault;
> > > int head, tail;
> > > + u64 iqe_err, ite_sid;
> > > + struct device *dev = NULL;
> > > + struct pci_dev *pdev = NULL;
> > > struct q_inval *qi = iommu->qi;
> > > int shift = qi_shift(iommu);
> > > @@ -1317,6 +1320,13 @@ static int qi_check_fault(struct intel_iommu *iommu, int index, int wait_index)
> > > tail = readl(iommu->reg + DMAR_IQT_REG);
> > > tail = ((tail >> shift) - 1 + QI_LENGTH) % QI_LENGTH;
> > > + /*
> > > + * SID field is valid only when the ITE field is Set in FSTS_REG
> > > + * see Intel VT-d spec r4.1, section 11.4.9.9
> > > + */
> > > + iqe_err = dmar_readq(iommu->reg + DMAR_IQER_REG);
> > > + ite_sid = DMAR_IQER_REG_ITESID(iqe_err);
> > > +
> > > writel(DMA_FSTS_ITE, iommu->reg + DMAR_FSTS_REG);
> > > pr_info("Invalidation Time-out Error (ITE) cleared\n");
> > > @@ -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"
>
> >
> > /*
> > * 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.
>
> >
> > > + 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.
>
> >
> > pdev = to_pci_dev(dev);
> > if (!pci_device_is_present(pdev))
> > return -ENODEV;
> >
> >
> > > + return -ETIMEDOUT;
> > -ENODEV.
>
> The ATS invalidation request could be sent from userland in later code,
> the userland code will care about the returned value, -ENODEV is one aspect
> of the fact (target device not present), while -ETIMEDOUT is another
> (timeout happened). we couldn't return them both.
>
> >
> > > + }
> > > if (qi->desc_status[wait_index] == QI_ABORT)
> > > return -EAGAIN;
> > > }
> > Sorry, again for nit picking a v13 patch. I'm not a domain expert but
> > this patchset seems reasonable to me.
>
> Though this is the v13, it is based on new rbtree code, you are welcome.
>
> Thanks,
> Ethan
>
> > regards,
> > dan carpenter
