linux-kernel - Re: [PATCH 00/12] KVM: MMU: do not unload MMU roots on all role changes

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Message-ID: <fc3a4cdc-5a88-55a9-cfcc-fb7936484cc8@redhat.com>
Date:   Wed, 9 Feb 2022 18:11:35 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        vkuznets@...hat.com, mlevitsk@...hat.com, dmatlack@...gle.com
Subject: Re: [PATCH 00/12] KVM: MMU: do not unload MMU roots on all role
 changes

On 2/9/22 18:07, Sean Christopherson wrote:
> On Wed, Feb 09, 2022, Paolo Bonzini wrote:
>> The TDP MMU has a performance regression compared to the legacy MMU
>> when CR0 changes often.  This was reported for the grsecurity kernel,
>> which uses CR0.WP to implement kernel W^X.  In that case, each change to
>> CR0.WP unloads the MMU and causes a lot of unnecessary work.  When running
>> nested, this can even cause the L1 to hardly make progress, as the L0
>> hypervisor it is overwhelmed by the amount of MMU work that is needed.
> 
> FWIW, my flushing/zapping series fixes this by doing the teardown in an async
> worker.  There's even a selftest for this exact case :-)
> 
> https://lore.kernel.org/all/20211223222318.1039223-1-seanjc@google.com

I'll check it out (it's next on my list as soon as I finally push 
kvm/{master,next}, which in turn was blocked by this work).

But not zapping the roots is even better---especially when KVM is nested 
and the TDP MMU's page table rebuild is very heavy on L0.  I'm not sure 
if there are any (cumulative) stats that capture the optimization, but 
if not I'll add them.

Paolo