Re: [PATCH v12 01/45] KVM: guest_memfd: Optimize away conversion overheads via dead-code elimination
From: Fuad Tabba <fuad.tabba@linux.dev>
Date: 2026-09-01 08:39:49
Also in:
kvm, linux-coco, linux-doc, linux-kselftest, linux-mm, lkml
On Mon, 31 Aug 2026 at 01:25, Ackerley Tng via B4 Relay [off-list ref] wrote:
From: Sean Christopherson <seanjc@google.com> 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 <redacted> Closes: https://lore.kernel.org/all/1ec08cd8-3072-4753-ad5e-cd34956647f8@linux.intel.com (local) Suggested-by: Ackerley Tng <redacted> Signed-off-by: Sean Christopherson <seanjc@google.com> Signed-off-by: Ackerley Tng <redacted>
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev> Cheers, /fuad
quoted hunk ↗ jump to hunk
--- 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_POPULATEdiff --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