On 3/22/2023 9:37 AM, Mathias Krause wrote:
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.
TTP --> TDP
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) {
Curiously, this patch only affects tdp_enabled, why does legacy MMU also
see comparable performance gains?
