Re: [PATCH v4 09/18] iommufd: Add a flag to skip clearing of IOPTE dirty

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Oct 18, 2023 at 09:27:06PM +0100, Joao Martins wrote:
> VFIO has an operation where it unmaps an IOVA while returning a bitmap with
> the dirty data. In reality the operation doesn't quite query the IO
> pagetables that the PTE was dirty or not. Instead it marks as dirty on
> anything that was mapped, and doing so in one syscall.
> 
> In IOMMUFD the equivalent is done in two operations by querying with
> GET_DIRTY_IOVA followed by UNMAP_IOVA. However, this would incur two TLB
> flushes given that after clearing dirty bits IOMMU implementations require
> invalidating their IOTLB, plus another invalidation needed for the UNMAP.
> To allow dirty bits to be queried faster, add a flag
> (IOMMU_GET_DIRTY_IOVA_NO_CLEAR) that requests to not clear the dirty bits
> from the PTE (but just reading them), under the expectation that the next
> operation is the unmap. An alternative is to unmap and just perpectually
> mark as dirty as that's the same behaviour as today. So here equivalent
> functionally can be provided with unmap alone, and if real dirty info is
> required it will amortize the cost while querying.

It seems fine, but I wonder is it really worthwhile? Did you measure
this? I suppose it is during the critical outage window

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason