diff mbox series

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

Message ID 20240710234222.2333120-10-jthoughton@google.com (mailing list archive)
State New, archived
Headers show
Series KVM: Post-copy live migration for guest_memfd | expand

Commit Message

James Houghton July 10, 2024, 11:42 p.m. UTC 
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 mbox series

Patch

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,