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Date: Mon, 26 Feb 2024 16:52:34 -0600
From: Bjorn Helgaas <helgaas@kernel.org>
To: Ethan Zhao <haifeng.zhao@linux.intel.com>
Cc: Dan Carpenter <dan.carpenter@linaro.org>, baolu.lu@linux.intel.com,
	bhelgaas@google.com, robin.murphy@arm.com, jgg@ziepe.ca,
	kevin.tian@intel.com, dwmw2@infradead.org, will@kernel.org,
	lukas@wunner.de, yi.l.liu@intel.com, iommu@lists.linux.dev,
	linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org
Subject: Re: [PATCH v13 3/3] iommu/vt-d: improve ITE fault handling if target
 device isn't valid
Message-ID: <20240226225234.GA211745@bhelgaas>
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In-Reply-To: <2d1788da-521c-4531-a159-81d2fb801d6c@linux.intel.com>

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