linux-kernel - Re: [PATCH v3 1/1] PCI: Add translated request only flag for pci_enable

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Date:   Wed, 1 Feb 2023 14:31:04 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     baolu.lu@...ux.intel.com, Bjorn Helgaas <bhelgaas@...gle.com>,
        Joerg Roedel <jroedel@...e.de>,
        Matt Fagnani <matt.fagnani@...l.net>,
        Christian König <christian.koenig@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Vasant Hegde <vasant.hegde@....com>,
        Tony Zhu <tony.zhu@...el.com>, linux-pci@...r.kernel.org,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/1] PCI: Add translated request only flag for
 pci_enable_pasid()

On 2023/2/1 8:14, Bjorn Helgaas wrote:
>>>> The AMD iommu driver allocates a new domain (called v2 domain) for the
>>> "v2 domain" needs to be something greppable -- an identifier,
>>> filename, etc.
>> The code reads,
>>
>> 2052         if (iommu_feature(iommu, FEATURE_GT) &&
>> 2053             iommu_feature(iommu, FEATURE_PPR)) {
>> 2054                 iommu->is_iommu_v2   = true;
>>
>> So, how about
>>
>> ..The AMD GPU has a private interface to its own AMD IOMMU, which could
>> be detected by the FEATURE_GT && FEATURE_PPR features. The AMD iommu
>> driver allocates a special domain for the GPU device ..
> Where is this special domain allocated?  I think the above tests for
> *IOMMU*  features (I assume "GTSup: Guest translations supported" and
> "PPRSup: Peripheral page request support" based on the AMD IOMMU
> spec).  It doesn't test that this is a GPU.

 From the discussion, my understanding is that the IOMMU is a GPU
dedicated IOMMU. The translated-request-only feature is enumerated
through the IOMMU feature bits. However, I am not familiar with AMD
architecture. AMD guys may have a better explanation.

> This change doesn't feel safe for all possible devices that have a
> PASID Capability because we don't know whether they*always*  use
> Translated addresses with PASID TLPs.

I tend to think you are right. :-)

Best regards,
baolu