[PATCH v11 01/46] KVM: guest_memfd: Optimize away conversion overheads via dead-code elimination

[Ackerley Tng <ackerleytng@google.com>]

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 <binbin.wu@linux.intel.com>
Closes: https://lore.kernel.org/all/1ec08cd8-3072-4753-ad5e-cd34956647f8@linux.intel.com
Suggested-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 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 589762140c3ef..8f8e5b6b70134 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);

-- 
2.55.0.887.g758fc8c411-goog