KVM: x86/mmu: KVM: x86/mmu: Skip unsync when large pages are allowed

Use the large-page metadata to avoid pointless attempts to search SP.

If the target GFN falls within a range where a large page is allowed,
then there cannot be a shadow page for that GFN; a shadow page in the
range would itself disallow using a large page. In that case, there
is nothing to unsync and mmu_try_to_unsync_pages() can return
immediately.

This is always true for TDP MMU without nested TDP, and holds for a
significant fraction of cases with shadow paging even all SPs are 4K.

For shadow paging, this optimization theoretically avoids work for about
1/e ~= 37% of GFNs, assuming one guest page table per 2M of memory and
that each GPT falls randomly into the 2M memory buckets. In a simple
test setup, it skipped unsync in a much higher percentage of cases,
mainly because the guest buddy allocator clusters GPTs into fewer
buckets.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Link: https://patch.msgid.link/20260123090304.32286-2-jiangshanlai@gmail.com
[sean: check for hugepage after write-tracking, update comment]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 733c1d5671cd..3dce38ffee76 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2941,6 +2941,15 @@ int mmu_try_to_unsync_pages(struct kvm *kvm, const struct kvm_memory_slot *slot,
 		return -EPERM;
 
 	/*
+	 * Only 4KiB mappings can become unsync, and KVM disallows hugepages
+	 * when accounting 4KiB shadow pages.  Upper-level gPTEs are always
+	 * write-protected (see above), thus if the gfn can be mapped with a
+	 * hugepage and isn't write-tracked, it can't have a shadow page.
+	 */
+	if (!lpage_info_slot(gfn, slot, PG_LEVEL_2M)->disallow_lpage)
+		return 0;
+
+	/*
 	 * The page is not write-tracked, mark existing shadow pages unsync
 	 * unless KVM is synchronizing an unsync SP.  In that case, KVM must
 	 * complete emulation of the guest TLB flush before allowing shadow