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@xxxxxxxxxx/ Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx> --- 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
