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 kalyazin@amazon.com
- 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 kalyazin@amazon.com
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