[PATCH v4 2/6] KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled

Mathias Krause
2023-03-22T01:34:41Z

There is no need to unload the MMU roots with TDP enabled when only
CR0.WP has changed -- the paging structures are still valid, only the
permission bitmap needs to be updated.

One heavy user of toggling CR0.WP is grsecurity's KERNEXEC feature to
implement kernel W^X.

The optimization brings a huge performance gain for this case as the
following micro-benchmark running 'ssdd 10 50000' from rt-tests[1] on a
grsecurity L1 VM shows (runtime in seconds, lower is better):

legacy TDP shadow
kvm-x86/next@d8708b 8.43s 9.45s 70.3s
+patch 5.39s 5.63s 70.2s

For legacy MMU this is ~36% faster, for TTP MMU even ~40% faster. Also
TDP and legacy MMU now both have a similar runtime which vanishes the
need to disable TDP MMU for grsecurity.

Shadow MMU sees no measurable difference and is still slow, as expected.

[1] https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git

Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
---
arch/x86/kvm/x86.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 237c483b1230..c6d909778b2c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -906,6 +906,18 @@ EXPORT_SYMBOL_GPL(load_pdptrs);

void kvm_post_set_cr0(struct kvm_vcpu *vcpu, unsigned long old_cr0, unsigned long cr0)
{
+ /*
+ * CR0.WP is incorporated into the MMU role, but only for non-nested,
+ * indirect shadow MMUs. If TDP is enabled, the MMU's metadata needs
+ * to be updated, e.g. so that emulating guest translations does the
+ * right thing, but there's no need to unload the root as CR0.WP
+ * doesn't affect SPTEs.
+ */
+ if (tdp_enabled && (cr0 ^ old_cr0) == X86_CR0_WP) {
+ kvm_init_mmu(vcpu);
+ return;
+ }
+
if ((cr0 ^ old_cr0) & X86_CR0_PG) {
kvm_clear_async_pf_completion_queue(vcpu);
kvm_async_pf_hash_reset(vcpu);
--
2.39.2