On Fri, Apr 08, 2022 at 08:21:55AM +0000, Tian, Kevin wrote: > (CC Jason Wang) > > > From: Jason Gunthorpe > > Sent: Thursday, April 7, 2022 11:24 PM > > > > While the comment was correct that this flag was intended to convey the > > block no-snoop support in the IOMMU, it has become widely implemented > > and > > used to mean the IOMMU supports IOMMU_CACHE as a map flag. Only the > > Intel > > driver was different. > > > > Now that the Intel driver is using enforce_cache_coherency() update the > > comment to make it clear that IOMMU_CAP_CACHE_COHERENCY is only > > about > > IOMMU_CACHE. Fix the Intel driver to return true since IOMMU_CACHE > > always > > works. > > > > The two places that test this flag, usnic and vdpa, are both assigning > > userspace pages to a driver controlled iommu_domain and require > > IOMMU_CACHE behavior as they offer no way for userspace to synchronize > > caches. > > > > Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx> > > Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx> > > btw the comment about vsnic and vdpa matches my thought. But > a recent change in Qemu [1] possibly wants confirmation from > Jason Wang. > > [1] https://lore.kernel.org/all/20220304133556.233983-20-mst@xxxxxxxxxx/ That patch seems to have run into the confusion this series is addressing. VFIO_DMA_CC_IOMMU and snoop control is absolutely not needed by VDPA. We expect the VDPA kernel driver to be well behaved and not cause its device to generate no-snoop TLPs. VDPA needs IOMMU_CACHE only. Jason
