Re: [PATCH v12 01/45] KVM: guest_memfd: Optimize away conversion overheads via dead-code elimination

[Fuad Tabba]

On Mon, 31 Aug 2026 at 01:25, Ackerley Tng via B4 Relay
<devnull+ackerleytng.google.com@xxxxxxxxxx> wrote:
>
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
>
> Add and use kvm_arch_has_gmem_convert() to guard guest_memfd's invocation
> of arch hooks related to converting memory between private and shared, as
> only one half of the x86 CoCo duo needs the runtime hooks (any pre-work is
> pure overhead for TDX).  At this exact moment, the overhead is negligible,
> but that will change when in-place conversion comes along, at which point
> to-shared conversions will "need" to find all affected folios prior to
> calling into arch code.  In quotes because very technically that work could
> be pushed to arch code, but that would bleed guest_memfd details into arch
> code and would be far worse than adding yet another kvm_arch_has... hook.
>
> Opportunistically provide the kvm_arch_gmem_make_private() declaration, and
> rely on dead-code elimination to eliminate the call to non-existent code
> when CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT=n.
>
> Reported-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/1ec08cd8-3072-4753-ad5e-cd34956647f8@xxxxxxxxxxxxxxx
> Suggested-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Reviewed-by: Fuad Tabba <fuad.tabba@xxxxxxxxx>

Cheers,
/fuad

> ---
>  arch/x86/include/asm/kvm_host.h | 3 +++
>  include/linux/kvm_host.h        | 3 ++-
>  virt/kvm/guest_memfd.c          | 5 ++---
>  3 files changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 683bb8bf43a94..6beddd8a0c749 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1854,6 +1854,9 @@ enum kvm_intr_type {
>  #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
>  #define kvm_arch_has_private_mem(kvm) ((kvm)->arch.has_private_mem)
>  #endif
> +#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
> +#define kvm_arch_has_gmem_convert() (!!kvm_x86_ops.gmem_make_private)
> +#endif
>
>  #define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 502465119ca0c..b7516e19ef419 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2597,9 +2597,10 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm,
>  }
>  #endif /* CONFIG_KVM_GUEST_MEMFD */
>
> -#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
>  int kvm_arch_gmem_make_private(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
>                                kvm_pfn_t nr_pages);
> +#ifndef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
> +#define kvm_arch_has_gmem_convert() false
>  #endif
>
>  #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_POPULATE
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 798d91df69537..2190e4d6b6bea 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -772,11 +772,10 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>                 folio_mark_uptodate(folio);
>         }
>
> -#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
> -       if (kvm_gmem_is_private_mem(file_inode(file), index))
> +       if (kvm_arch_has_gmem_convert() &&
> +           kvm_gmem_is_private_mem(file_inode(file), index))
>                 r = kvm_arch_gmem_make_private(kvm, gfn, *pfn,
>                                                (kvm_pfn_t)1 << *max_order);
> -#endif
>
>         folio_unlock(folio);
>         folio_put(folio);
>
> --
> 2.55.0.897.gb25b4bd76c-goog
>
>