Commit b3ae3ceb authored by Lai Jiangshan's avatar Lai Jiangshan Committed by Sean Christopherson
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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: default avatarLai 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: default avatarSean Christopherson <seanjc@google.com>
parent ecb80629

arch/x86/kvm/mmu/mmu.c

+9 −0
Original line number Diff line number Diff line
@@ -2940,6 +2940,15 @@ int mmu_try_to_unsync_pages(struct kvm *kvm, const struct kvm_memory_slot *slot, 
	if (kvm_gfn_is_write_tracked(kvm, slot, gfn))

		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