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

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Message-ID: <039a19e5-d1ff-47ae-aa35-3347c08acc13@moroto.mountain>
Date: Fri, 23 Feb 2024 09:08:51 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Ethan Zhao <haifeng.zhao@...ux.intel.com>
Cc: baolu.lu@...ux.intel.com, bhelgaas@...gle.com, robin.murphy@....com,
	jgg@...pe.ca, kevin.tian@...el.com, dwmw2@...radead.org,
	will@...nel.org, lukas@...ner.de, yi.l.liu@...el.com,
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH v13 3/3] iommu/vt-d: improve ITE fault handling if target
 device isn't valid

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