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

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Date:   Thu, 30 Nov 2023 12:18:27 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Baolu Lu <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 Thu, Nov 30, 2023 at 01:44:19PM +0800, Baolu Lu wrote:
> On 2023/11/30 4:13, Jason Gunthorpe wrote:
> > 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?
> 
> This fix only affects the non-scalable mode that doesn't support PASID
>  features.

Ah, OK so it is OK. I'm glad the new mode supports ATS against
passthrough.

Thanks,
Jason