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| author | Lai Jiangshan <jiangshan.ljs@antgroup.com> | 2026-01-23 17:03:03 +0800 |
|---|---|---|
| committer | Sean Christopherson <seanjc@google.com> | 2026-03-12 10:36:01 -0700 |
| commit | b3ae3ceb556945724d0c046ddb4ea0cf492a0ce6 (patch) | |
| tree | 3df01ebfdf04a8e792cc2388e6c5f45170908156 /arch/x86 | |
| parent | ecb80629321306547f7ad13b0ca5ef9cf8cdbb77 (diff) | |
| download | linux-next-b3ae3ceb556945724d0c046ddb4ea0cf492a0ce6.tar.gz linux-next-b3ae3ceb556945724d0c046ddb4ea0cf492a0ce6.zip | |
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>
Diffstat (limited to 'arch/x86')
| -rw-r--r-- | arch/x86/kvm/mmu/mmu.c | 9 |
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 |