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

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Date: Thu, 20 Jun 2024 08:50:13 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 Kevin Tian <kevin.tian@...el.com>,
 Jacek Lawrynowicz <jacek.lawrynowicz@...ux.intel.com>,
 iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] iommu/vt-d: Fix missed device TLB cache tag

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.

A possible solution is to move ATS enabling to a place before the
default domain attachment. However, this is not future-proof,
considering that we will eventually hand over the ATS control to the
device drivers. Therefore, this fix creates the device TLB cache tags as
long as ats_supported is true and relies on ats_enabled to decide
whether device TLB needs to be invalidated.

Best regards,
baolu