Xu Yilun <yilun.xu@xxxxxxxxxxxxxxx> writes: > 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. I see, thanks for explaining. > 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. > If by the time KVM gets the conversion request, the page is unpinned, then we're all good, right? When guest_memfd gets the conversion request, as part of conversion handling it will request to zap the page from stage-2 page tables. TDX module would see that the page is unpinned and the unmapping will proceed fine. Is that understanding correct? >> 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. > On this note, I'd also like to check something else. Putting TDX connect and IOMMUs aside, if the host unmaps a guest private page today without the guest requesting it, the unmapping will work and the guest will be broken, right? > 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".
