On Wed, Jul 16, 2025 at 03:22:06PM -0700, Ackerley Tng wrote: > Yan Zhao <yan.y.zhao@xxxxxxxxx> writes: > > > 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. > > 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. > > > > > > Yan, Yilun, would it work if, on conversion, > > 1. guest_memfd notifies IOMMU that a conversion is about to happen for a > PFN range It is the Guest fw call to release the pinning. By the time VMM get the conversion requirement, the page is already physically unpinned. So I agree with Jason the pinning doesn't have to reach to iommu from SW POV. > 2. IOMMU forwards the notification to TDX code in the kernel > 3. TDX code in kernel tells TDX module to stop thinking of any PFNs in > the range as pinned for DMA? TDX host can't stop the pinning. Actually this mechanism is to prevent host from unpin/unmap the DMA out of Guest expectation. Thanks, Yilun > > If the above is possible then by the time we get to unmapping from > S-EPTs, TDX module would already consider the PFNs in the range "not > pinned for DMA".
