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@xxxxxxxxxx> Jason
