linux-kernel - RE: [PATCH 1/1] iommu/vt-d: Fix missed device TLB cache tag

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Date: Thu, 20 Jun 2024 03:57:38 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, Jason Gunthorpe <jgg@...pe.ca>
CC: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, "Robin
 Murphy" <robin.murphy@....com>, Jacek Lawrynowicz
	<jacek.lawrynowicz@...ux.intel.com>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/1] iommu/vt-d: Fix missed device TLB cache tag

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Thursday, June 20, 2024 11:14 AM
> 
> On 6/20/24 11:04 AM, Tian, Kevin wrote:
> >> From: Baolu Lu<baolu.lu@...ux.intel.com>
> >> Sent: Thursday, June 20, 2024 8:50 AM
> >>
> >> On 6/20/24 12:46 AM, Jason Gunthorpe wrote:
> >>> On Wed, Jun 19, 2024 at 09:53:45AM +0800, Lu Baolu wrote:
> >>>> When a domain is attached to a device, the required cache tags are
> >>>> assigned to the domain so that the related caches could be flushed
> >>>> whenever it is needed. The device TLB cache tag is created selectively
> >>>> by checking the ats_enabled field of the device's iommu data. This
> >>>> creates an ordered dependency between attach and ATS enabling paths.
> >>>>
> >>>> The device TLB cache tag will not be created if device's ATS is enabled
> >>>> after the domain attachment. This causes some devices, for example
> >>>> intel_vpu, to malfunction.
> >>> What? How is this even possible?
> >>>
> >>> ATS is controlled exclusively by the iommu driver, how can it be
> >>> changed without the driver knowing??
> >> Yes. ATS is currently controlled exclusively by the iommu driver. The
> >> intel iommu driver enables PCI/ATS on the probe path after the default
> >> domain is attached. That means when the default domain is attached to
> >> the device, the ats_supported is set, but ats_enabled is cleared. So the
> >> cache tag for the device TLB won't be created.
> > I don't quite get why this is specific to the probe path and the default
> > domain.
> 
> The issue is with the domain attaching device path, not specific to the
> probe or default domain.
> 
> >
> > dmar_domain_attach_device()
> > {
> > 	cache_tag_assign_domain();
> > 	//setup pasid entry for pt/1st/2nd
> > 	iommu_enable_pci_caps();
> > }
> >
> > seems that for all domain attaches above is coded in a wrong order
> > as ats is enabled after the cache tag is assigned.
> 
> Yes, exactly. But simply changing the order isn't future-proof,
> considering ATS control will eventually be moved out of iommu drivers.

I'm not sure the way this patch goes is future-proof either. Ideally the devtlb
cache tag should always be assigned together with enabling the ats capability
then in that case we won't have a case assigning a tag upon info->ats_supported
at attach and then check info->ats_enabled run-time for flush.

and order-wise it makes slightly more sense to assign cache tag at end of 
the attach function after all other configurations have been completed.

with that I prefer to a simple fix by reverting the order instead of adding
unnecessary run-time overhead for unclear future. 😊

> 
> > why is it considered
> > to affect only some devices e.g. intel_vpu?
> 
> This bug was discovered during testing of the intel_vpu device and
> affects devices other than the intel_vpu. The commit message is a bit
> confusing. :-)
> 
> Best regards,
> baolu