On Wed, Jul 2, 2025 at 1:38 AM Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote: > > On Tue, Jun 24, 2025 at 07:10:38AM -0700, Vishal Annapurve wrote: > > On Tue, Jun 24, 2025 at 6:08 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > > > > > On Tue, Jun 24, 2025 at 06:23:54PM +1000, Alexey Kardashevskiy wrote: > > > > > > > Now, I am rebasing my RFC on top of this patchset and it fails in > > > > kvm_gmem_has_safe_refcount() as IOMMU holds references to all these > > > > folios in my RFC. > > > > > > > > So what is the expected sequence here? The userspace unmaps a DMA > > > > page and maps it back right away, all from the userspace? The end > > > > result will be the exactly same which seems useless. And IOMMU TLB > > > > As Jason described, ideally IOMMU just like KVM, should just: > > 1) Directly rely on guest_memfd for pinning -> no page refcounts taken > > by IOMMU stack > In TDX connect, TDX module and TDs do not trust VMM. So, it's the TDs to inform > TDX module about which pages are used by it for DMAs purposes. > So, if a page is regarded as pinned by TDs for DMA, the TDX module will fail the > unmap of the pages from S-EPT. > > If IOMMU side does not increase refcount, IMHO, some way to indicate that > certain PFNs are used by TDs for DMA is still required, so guest_memfd can > reject the request before attempting the actual unmap. So it looks like guest_memfd will need an interface with KVM/IOMMU backends to check if unmapping can succeed. And if unmapping still fails, there should be a way for KVM/IOMMU backends to kill the TD and any TDIs bound to that TD. > Otherwise, the unmap of TD-DMA-pinned pages will fail. > > Upon this kind of unmapping failure, it also doesn't help for host to retry > unmapping without unpinning from TD. >
