linux-kernel - Re: [GIT PULL] KVM: x86: MMU changes for 6.9

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Message-ID: <ZfNEEFmTkx-RVuix@google.com>
Date: Thu, 14 Mar 2024 11:38:08 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] KVM: x86: MMU changes for 6.9

On Thu, Mar 14, 2024, Paolo Bonzini wrote:
> On Fri, Mar 8, 2024 at 11:37 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> >  - Zap TDP MMU roots at 4KiB granularity to minimize the delay in yielding if
> >    a reschedule is needed, e.g. if a high priority task needs to run.  Because
> >    KVM doesn't support yielding in the middle of processing a zapped non-leaf
> >    SPTE, zapping at 1GiB granularity can result in multi-millisecond lag when
> >    attempting to schedule in a high priority.
> >
> 
> Would 2 MiB provide a nice middle ground?

Not really?

Zapping at 2MiB definitely fixes the worst of the tail latencies, but there is
still a measurable difference between 2MiB and 4KiB.  And on the other side of the
coing, I was unable to observe a meaningful difference in total runtime by zapping
at 2MiB, or even 1GiB, versus 4KiB.

In other words, AFAICT, there's no need to shoot for a middle ground because trying
to zap at larger granularities doesn't buy us anything.