Re: [PATCH v7 13/24] iommu/arm-smmu-v3: Enable broadcast TLB maintenance

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On 2020-05-21 15:17, Will Deacon wrote:
[+Marc]

On Tue, May 19, 2020 at 07:54:51PM +0200, Jean-Philippe Brucker wrote:
The SMMUv3 can handle invalidation targeted at TLB entries with shared
ASIDs. If the implementation supports broadcast TLB maintenance, enable it and keep track of it in a feature bit. The SMMU will then be affected by
inner-shareable TLB invalidations from other agents.

A major side-effect of this change is that stage-2 translation contexts are now affected by all invalidations by VMID. VMIDs are all shared and the only ways to prevent over-invalidation, since the stage-2 page tables are not shared between CPU and SMMU, are to either disable BTM or allocate
different VMIDs. This patch does not address the problem.

This sounds like a potential performance issue, particularly as we expose stage-2 contexts via VFIO directly. Maybe we could reserve some portion of
VMID space for the SMMU? Marc, what do you reckon?

Certainly doable when we have 16bits VMIDs. With smaller VMID spaces (like on v8.0), this is a bit more difficult (we do have pretty large v8.0 systems
around). How many VMID bits are we talking about?

        M.
--
Jazz is not dead. It just smells funny...




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