On Fri, Jul 18, 2025 at 05:15:42PM +0800, Xu Yilun wrote: > > I would say this discussion is irrelevant to this case since we are > > not doing any kind of in-place conversion. > > > > 1) At VM start the VMM gets the shared DMABUF and maps it to IOAS and > > EPT > > 2) Bind request comes in, VMM unmaps shared DMABUF from IOAS and EPT > > then closes it. > > 3) Bind is done > > 4) VMM opens a shared and private DMABUF FD and learns the valid > > ranges in both DMABUFs (ie what PFNS are private/shared) > > Who decides the dmabuf be shared or private? I assume you mean > userspace, VFIO doesn't know about the shareability of each dmabuf? I imagined userspace would put a flag in the ioctl to VFIO to get the dmabuf kind it wants. VFIO would have to be told from iommufd what the per-pfn shared/private layout is and can use that to manage the dmabufs. > > 5) VMM maps the shared DMABUF fragments to the EPT and IOAS > > Userspace could set a shared only memory slot. > > > 6) VMM maps the private DMABUF fragments to the S-EPT > > Userspace could set a private only memory slot, but maybe the concern > is KVM can't trust userspace about the assertion of private. I imagined kvm can query the dmabuf and learn it is all private from VFIO. The whole dmabuf, not per-pfn. Same for #5 it can check it is shared memory too. I sort of imagine using the same mechanism for p2p where we can mark memory in the dmabuf with a 'provider' indicating these details. We will have to see. > If VFIO should be aware of the shareability, IOMMUFD to VFIO callback is > needed after TSM bind. Yes, whever this list of per-PFN shared/private must be shared between VFIO and the TSM. > It is simpler on kernel side. FYR, may introduce some complexity in > userspace, this may result in 3+ dmabufs. There may be shared holes > in the BAR, and kvm slot can't be gfn sparse. This may not be a big > problem. Userspace probably needs to create multiple slots for sparsity Again this seems much better if userspace handles it and just uses simple mostly existing KVM interfaces. Why 3+ dmabufs? Isn't just one shared and one private per BAR? Jason
