On 07.05.23 09:32, Robert Hoo wrote: > 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 Thanks, Sean fixed it up in the final commit: https://git.kernel.org/linus/01b31714bd90 > >> 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? Because 'tdp_enabled' just implies EPT / NPT and only 'tdp_mmu_enabled' decides which MMU mode to use -- either legacy or TDP MMU (see kvm_configure_mmu() and now gets invoked from vmx.c / svm.c). Thanks, Mathias
