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Date: Tue, 27 Feb 2024 10:29:54 -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: <20240227162954.GA239330@bhelgaas>
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In-Reply-To: <ae9137f1-3e9b-4d84-956f-4e8c31d2e1bb@linux.intel.com>

On Tue, Feb 27, 2024 at 10:30:36AM +0800, Ethan Zhao wrote:
> On 2/27/2024 6:52 AM, Bjorn Helgaas wrote:
> > 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.
> 
> I meant "ATS Invalidation Request" here is one term in PCIe spec, 'valid'
> is used to describe the word 'target'.
> 
> This patch isn't intended to work as the same logic as patch [2/3], this
> aims to break the blindly dead loop not to retry the timeout request after
> ITE fault happened.
> 
> > 
> > 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.
> 
> Here we check to see if the ITE fault was caused by device is not present.
> The opposite logic, not predict the future, but find the cause of the fault
> already happened, if pci_device_is_present() tells us the device isn't
> there, it is reliable I think.
> 
> > 
> > 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
> 
> There is a dead loop here to blindly retry to timeout request if
> ITE happened, we want to break that loop if the target device was
> gone.
> 
> > 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
> 
> That logic works for simple safe /surprise removal as described in
> patch[2/3], no race there that case at all.
> 
> > 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
> 
> We are not relying on pci_device_is_present() here in this patch to close
> the race window between aggressive surprise removal and ATS invalidation
> Request, we are doing post-fault handling here.

OK, sorry, I guess I missed that this fixes the code that handles the
Completion Timeouts.

> > 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