On Fri, Oct 24, 2025, Nikita Kalyazin wrote: > > > On 23/10/2025 17:07, Sean Christopherson wrote: > > On Mon, Oct 20, 2025, Nikita Kalyazin wrote: > > > From: Nikita Kalyazin <[email protected]> > > + Vishal and Ackerley > > > > > > > write syscall populates guest_memfd with user-supplied data in a generic > > > way, ie no vendor-specific preparation is performed. If the request is > > > not page-aligned, the remaining bytes are initialised to 0. > > > > > > write is only supported for non-CoCo setups where guest memory is not > > > hardware-encrypted. > > > > Please include all of the "why". The code mostly communicates the "what", > > but > > it doesn't capture why write() support is at all interesting, nor does it > > explain > > why read() isn't supported. > > Hi Sean, > > Thanks for the review. > > Do you think including the explanation from the cover letter would be > sufficient?
It's pretty close. A few more details would be helpful, e.g. to explain that VMMs may use write() to populate the initial image > Shall I additionally say that read() isn't supported because there is no use > case for it as of now or would it be obvious? Hmm, I think if you want to exclude read() support, the changelog should explicitly state why. E.g. "there's no use case" is quite different from "deliberately don't support read() for security reasons". > > > Signed-off-by: Nikitia Kalyazin <[email protected]> > > > --- > > > virt/kvm/guest_memfd.c | 48 ++++++++++++++++++++++++++++++++++++++++++ > > > > There's a notable lack of uAPI and Documentation chanegs. I.e. this needs a > > GUEST_MEMFD_FLAG_xxx along with proper documentation. > > Would the following be ok in the doc? > > When the capability KVM_CAP_GUEST_MEMFD_WRITE is supported, the 'flags' No capability is necessary, see d2042d8f96dd ("KVM: Rework KVM_CAP_GUEST_MEMFD_MMAP into KVM_CAP_GUEST_MEMFD_FLAGS"). > field > supports GUEST_MEMFD_FLAG_WRITE. Setting this flag on guest_memfd creation > enables write() syscall operations to populate guest_memfd memory from host > userspace. > > When a write() operation is performed on a guest_memfd file descriptor with > the > GUEST_MEMFD_FLAG_WRITE set, the syscall will populate the guest memory with > user-supplied data in a generic way, without any vendor-specific > preparation. > The write operation is only supported for non-CoCo (Confidential Computing) > setups where guest memory is not hardware-encrypted. The restriction should be that guest memory must be SHARED, i.e. not PRIVATE. Strictly speaking, guest memory can be encrypted, e.g. with SME and TME (I think TME is still a thing?), but with a shared key and thus accessible from the host. Even if that weren't the case, we want to support this for CoCo VMs. > If the write request is not page-aligned, any remaining bytes within the page > are initialized to zero. Why? (Honest question, e.g. is that standard file semantics?) > > And while it's definitely it's a-ok to land .write() in advance of the > > direct map > > changes, we do need to at least map out how we want the two to interact, > > e.g. so > > that we don't end up with constraints that are impossible to satisfy. > > > > write() shall not attempt to access a page that is not in the direct map, > which I believe can be achieved via kvm_kmem_gmem_write_begin() consulting > the KVM_GMEM_FOLIO_NO_DIRECT_MAP in folio->private (introduced in [1]). > > Do you think we should mention it in the commit message in some way? What > particular constraint are you cautious about? I want to be cautious with respect to the ABI/uAPI. Patrick's series also adds a flag, and guest_memfd doesn't currently provide a way to toggle flags after the file is created. That begs the question of how GUEST_MEMFD_FLAG_NO_DIRECT_MAP will co-exist with GUEST_MEMFD_FLAG_WRITE. Presumably the goal is to use write() to initialize memory, and _then_ nuke the direct map. I want line of sight to understanding the exact semantics/flows. E.g. will KVM require userspace to clear GUEST_MEMFD_FLAG_WRITE before allowing NO_DIRECT_MAP? Or will the write() simply fail? How will the sequencing be achieved? > > > + struct inode *inode = file_inode(file); > > > + pgoff_t index = pos >> PAGE_SHIFT; > > > + struct folio *folio; > > > + > > > + if (!kvm_gmem_supports_mmap(inode)) > > > > Checking for MMAP is neither sufficient nor strictly necessary. MMAP > > doesn't > > imply SHARED, and it's not clear to me that mmap() support should be in any > > way > > tied to WRITE support. > > As in my reply to the comment about doc, I plan to introduce > KVM_CAP_GUEST_MEMFD_WRITE and GUEST_MEMFD_FLAG_WRITE. The > kvm_arch_supports_gmem_write() will be a weak symbol and relying on > !kvm_arch_has_private_mem() on x86, similar to > kvm_arch_supports_gmem_mmap(). Does it look right? No. As above, write() should be allowed iff memory is SHARED. Relevant commits that are now in Linus' tree: 44c6cb9fe9888b371e31165b2854bd0f4e2787d4 KVM: guest_memfd: Allow mmap() on guest_memfd for x86 VMs with private memory 9aef71c892a55e004419923ba7129abe3e58d9f1 KVM: Explicitly mark KVM_GUEST_MEMFD as depending on KVM_GENERIC_MMU_NOTIFIER 5d3341d684be80892d8f6f9812f90f9274b81177 KVM: guest_memfd: Invalidate SHARED GPAs if gmem supports INIT_SHARED
