Messages in this thread | | | Date | Sun, 7 May 2023 15:32:21 +0800 | Subject | Re: [PATCH v4 2/6] KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled | From | Robert Hoo <> |
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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?
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