[RFC PATCH 09/18] KVM: x86: Add vCPU fault fast-path for Userfault

[James Houghton <jthoughton@google.com>]

Without this fast-path, we will take the asynchronous userfault path
every time, which is inefficient.

As implemented today, KVM Userfault isn't well optimized at all, but I'm
providing this optimization because something like this will be required
to significantly improve post-copy performance. Memory fault exits for
userfaultfd were proposed for the same reason[1].

[1]: https://lore.kernel.org/kvm/20240215235405.368539-7-amoorthy@google.com/

Signed-off-by: James Houghton <jthoughton@google.com>
---
 arch/x86/kvm/mmu/mmu.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6b6a053758ec..f0dbc3c68e5c 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4380,6 +4380,13 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 	if (fault->is_private)
 		return kvm_faultin_pfn_private(vcpu, fault);
 
+	/* Pre-check for userfault and bail out early. */
+	if (gfn_has_userfault(fault->slot->kvm, fault->gfn)) {
+		fault->pfn = KVM_PFN_ERR_USERFAULT;
+		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
+		return -EFAULT;
+	}
+
 	async = false;
 	fault->pfn = __gfn_to_pfn_memslot(fault->slot, fault->gfn, false, false,
 					  &async, fault->write,
-- 
2.45.2.993.g49e7a77208-goog