Re: [PATCH RFC v1 3/5] KVM: guest_memfd: Call arch invalidation hooks when converting to shared

[Vishal Annapurve <vannapurve@google.com>]

On Thu, Jun 12, 2025 at 5:56 PM Michael Roth <michael.roth@amd.com> wrote:
>
> When guest_memfd is used for both shared/private memory, converting
> pages to shared may require kvm_arch_gmem_invalidate() to be issued to
> return the pages to an architecturally-defined "shared" state if the
> pages were previously allocated and transitioned to a private state via
> kvm_arch_gmem_prepare().
>
> Handle this by issuing the appropriate kvm_arch_gmem_invalidate() calls
> when converting ranges in the filemap to a shared state.
>
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> ---
>  virt/kvm/guest_memfd.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index b77cdccd340e..f27e1f3962bb 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -203,6 +203,28 @@ static int kvm_gmem_shareability_apply(struct inode *inode,
>         struct maple_tree *mt;
>
>         mt = &kvm_gmem_private(inode)->shareability;
> +
> +       /*
> +        * If a folio has been allocated then it was possibly in a private
> +        * state prior to conversion. Ensure arch invalidations are issued
> +        * to return the folio to a normal/shared state as defined by the
> +        * architecture before tracking it as shared in gmem.
> +        */
> +       if (m == SHAREABILITY_ALL) {
> +               pgoff_t idx;
> +
> +               for (idx = work->start; idx < work->start + work->nr_pages; idx++) {

It is redundant to enter this loop for VM variants that don't need
this loop e.g. for pKVM/TDX. I think KVM can dictate a set of rules
(based on VM type) that guest_memfd will follow for memory management
when it's created, e.g. something like:
1) needs pfn invalidation
2) needs zeroing on shared faults
3) needs zeroing on allocation

> +                       struct folio *folio = filemap_lock_folio(inode->i_mapping, idx);
> +
> +                       if (!IS_ERR(folio)) {
> +                               kvm_arch_gmem_invalidate(folio_pfn(folio),
> +                                                        folio_pfn(folio) + folio_nr_pages(folio));
> +                               folio_unlock(folio);
> +                               folio_put(folio);
> +                       }
> +               }
> +       }
> +
>         return kvm_gmem_shareability_store(mt, work->start, work->nr_pages, m);
>  }
>
> --
> 2.25.1
>