linux-kernel - Re: [PATCH 1/1] iommu/vt-d: Disable PCI ATS in legacy passthrough mode

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Date:   Wed, 29 Nov 2023 16:13:24 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Tian <kevin.tian@...el.com>, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] iommu/vt-d: Disable PCI ATS in legacy passthrough
 mode

On Tue, Nov 14, 2023 at 09:10:35AM +0800, Lu Baolu wrote:
> When IOMMU hardware operates in legacy mode, the TT field of the context
> entry determines the translation type, with three supported types (Section
> 9.3 Context Entry):
> 
> - DMA translation without device TLB support
> - DMA translation with device TLB support
> - Passthrough mode with translated and translation requests blocked
> 
> Device TLB support is absent when hardware is configured in passthrough
> mode.
> 
> Disable the PCI ATS feature when IOMMU is configured for passthrough
> translation type in legacy (non-scalable) mode.

Oh.. That is the same horrible outcome that ARM has :(

The issue is what to do if the RID translation is in identity but a
PASID is attached that should be using ATS - eg do you completely
loose SVA support if the RID is set to the optimized identity mode?

I vote no. We should make the drivers aware that they should not use
ATS on their RIDs instead :(

Jason