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

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Date: Thu, 20 Jun 2024 11:24:00 +0530
From: Vasant Hegde <vasant.hegde@....com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, "Tian, Kevin"
 <kevin.tian@...el.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

Hi Baolu,

On 6/20/2024 8:43 AM, Baolu Lu wrote:
> 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.

[Unrelated to this patch]

You mean ATS setup will be moved to individual device driver? Is there any
reason for that?


-Vasant