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Date:   Tue, 14 Nov 2023 03:16:27 +0000
From:   "Tian, Kevin" <>
To:     Lu Baolu <>,
        Joerg Roedel <>,
        "Will Deacon" <>,
        Robin Murphy <>,
        "Jason Gunthorpe" <>
CC:     "" <>,
        "" <>
Subject: RE: [PATCH 1/1] iommu/vt-d: Support enforce_cache_coherency only for
 empty domains

> From: Lu Baolu <>
> Sent: Tuesday, November 14, 2023 9:11 AM
> The enforce_cache_coherency callback ensures DMA cache coherency for
> devices attached to the domain.
> Intel IOMMU supports enforced DMA cache coherency when the Snoop
> Control bit in the IOMMU's extended capability register is set.
> Supporting it differs between legacy and scalable modes.
> In legacy mode, it's supported page-level by setting the SNP field
> in second-stage page-table entries. In scalable mode, it's supported
> in PASID-table granularity by setting the PGSNP field in PASID-table
> entries.
> In legacy mode, mappings before attaching to a device have SNP
> fields cleared, while mappings after the callback have them set.
> This means partial DMAs are cache coherent while others are not.
> One possible fix is replaying mappings and flipping SNP bits when
> attaching a domain to a device. But this seems to be over-engineered,
> given that all real use cases just attach an empty domain to a device.
> To meet practical needs while reducing mode differences, only support
> enforce_cache_coherency on a domain without mappings if SNP field is
> used.
> Fixes: fc0051cb9590 ("iommu/vt-d: Check domain force_snooping against
> attached devices")
> Signed-off-by: Lu Baolu <>

Reviewed-by: Kevin Tian <>

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