Re: [PATCH v2] iommu/arm-smmu-v3: Add a user-configurable tlb_invalidate_threshold

[Nicolin Chen]

On Wed, Aug 16, 2023 at 01:43:50PM -0700, Nicolin Chen wrote:
 
> When receiving an __arm_smmu_tlb_inv_range() call with a large size, there
> could be a long latency at this function call: one part is coming from a
> large software overhead in the routine of building commands, and the other
> part is coming from CMDQ hardware consuming the large number of commands.
> This latency could be significantly large on an SMMU that does not support
> range invalidation commands, i.e. no ARM_SMMU_FEAT_RANGE_INV.
> 
> One way to optimize this is to replace a large number of VA invalidation
> commands with one single per-asid invalidation command, when the requested
> size reaches a threshold. This threshold can be configurable depending on
> the SMMU implementaion.

I'm rethinking about this size-based threshold, since what really
affects the latency is the number of the invalidation commands in
the request. So having an npages-based threshold might be optimal,
though the idea and implementation would be similar.

Thanks
Nicolin