[RFC PATCH 11/18] KVM: arm64: Add vCPU memory fault fast-path for Userfault

James Houghton
2024-07-10T23:42:49Z

Make this optimization for the same reason we make it for x86: because
it necessary for sufficient post-copy performance when scaling up to
hundreds of cores (even though KVM Userfault today doesn't scale very
well).

Signed-off-by: James Houghton <jthoughton@google.com>
---
arch/arm64/kvm/mmu.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index ac283e606516..c84633c9ab98 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1488,6 +1488,15 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
mmu_seq = vcpu->kvm->mmu_invalidate_seq;
mmap_read_unlock(current->mm);

+ if (gfn_has_userfault(memslot->kvm, gfn)) {
+ kvm_prepare_memory_fault_exit(vcpu, gfn << PAGE_SHIFT,
+ PAGE_SIZE, write_fault,
+ /*exec=*/false,
+ /*private=*/false,
+ /*userfault=*/true);
+ return -EFAULT;
+ }
+
pfn = __gfn_to_pfn_memslot(memslot, gfn, false, false, NULL,
write_fault, &writable, NULL);
if (pfn == KVM_PFN_ERR_HWPOISON) {
--
2.45.2.993.g49e7a77208-goog