Re: [PATCH v3 3/6] KVM: x86: Do not unload MMU roots when only toggling CR0.WP

[Mathias Krause]

On 15.03.23 23:11, Sean Christopherson wrote:
> Can you tweak the shortlog to something like this?  I want to make it very clear
> that this applies only to the TDP case (more below).  I did a double take when I
> first read the subject :-)
> 
>   KVM: x86: Do not unload MMU roots when only toggling CR0.WP with TDP enabled

Sure, will do!

> 
> On Wed, Feb 01, 2023, Mathias Krause wrote:
>> There is no need to unload the MMU roots for a direct MMU role 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.git/queue          11.55s   13.91s    75.2s
>> kvm.git/queue+patch     7.32s    7.31s    74.6s
>>
>> For legacy MMU this is ~36% faster, for TTP MMU even ~47% faster. Also
>> TDP and legacy MMU now both have around the same 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@xxxxxxxxxx>
> 
> No need for this, I just threw a snippet at you as part of code review.  And IMO,
> if someone goes through the pain of running benchmarks, they get full credit no
> matter what ;-)

Reviewers (and in your case maintainers) get far too little credit, so
I'd rather keep that tag, if you don't mind that hard ;)

> 
>> Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
>> ---
>> v2: handle the CR0.WP case directly in kvm_post_set_cr0() and only for
>> the direct MMU role -- Sean
>>
>> I re-ran the benchmark and it's even faster than with my patch, as the
>> critical path is now the first one handled and is now inline. Thanks a
>> lot for the suggestion, Sean!
>>
>>  arch/x86/kvm/x86.c | 9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 508074e47bc0..f09bfc0a3cc1 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -902,6 +902,15 @@ EXPORT_SYMBOL_GPL(load_pdptrs);
>>  
>>  void kvm_post_set_cr0(struct kvm_vcpu *vcpu, unsigned long old_cr0, unsigned long cr0)
>>  {
>> +	/*
>> +	 * Toggling just CR0.WP doesn't invalidate page tables per se, only the
>> +	 * permission bits.
>> +	 */
>> +	if (vcpu->arch.mmu->root_role.direct && (cr0 ^ old_cr0) == X86_CR0_WP) {
> 
> Past me was wrong, which is a very good thing in this case.  Per the APM,
> 
>   The host hCR0.WP bit is ignored under nested paging.
> 

See what you did? You even went that far and re-read the manual.
Definitely deserves credit!

> which means that CR0.WP doesn't need to be incorporated into the role.  Ha!  And
> really-past me even wrote a very nice comment to call that out in commit 31e96bc63655
> ("KVM: nSVM: Add a comment to document why nNPT uses vmcb01, not vCPU state").
> 
> Double ha!  That's all moot, because if this code is reached for a nested MMU,
> it means L2 is active and the CR0 being changed is gCR0, not L1's hCR0.
> 
> So more simply, this can be
> 
> 	if (tdp_enabled && (cr0 ^ old_cr0) == X86_CR0_WP)

Looks much simpler, indeed. But might deserve a little comment itself
why it's fine test 'tdp_enabled' only...

> 
> or if we want to exempt non-paging mode for the shadow MMU as well...
> 
> 	if ((cr0 ^ old_cr0) == X86_CR0_WP && (tdp_enabled || !(cr0 & X86_CR0_PG)))
> 
> Actually, if we bother to check CR0.PG, then we might as well get greedy and
> skip _all_ updates when paging is disabled.  E.g. end up with this over two
> patches?  First one exempts the tdp_enabled case, second one completely exempts
> paging disabled.
> 

...and there it is already! :D

> 	/*
> 	 * CR0.WP is incorporated into the MMU role, but only for non-nested,
> 	 * indirect shadow MMUs.  If paging is disabled, no updates are needed
> 	 * as there are no permission bits to emulate.  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 when TDP is enabled.
> 	 */
> 	if ((cr0 ^ old_cr0) == X86_CR0_WP) {
> 		if (!(cr0 & X86_CR0_PG))
> 			return;
> 
> 		if (tdp_enabled) {
> 			kvm_init_mmu(vcpu);
> 			return;
> 		}
> 	}

Thanks, Sean! Sounds all very good to me. I'll cook up these commits, do
some more tests and send a v4 tomorrow, if time allows.