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

Mathias Krause
2023-05-08T09:31:20Z

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