linux-kernel - Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO

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Date:   Thu, 17 Aug 2017 11:00:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS

On 17/08/2017 00:31, Michael S. Tsirkin wrote:
> On Wed, Aug 16, 2017 at 11:25:35PM +0200, Paolo Bonzini wrote:
>> Yes, I agree.  EMULTYPE_SKIP is fine because failed decoding still
>> causes an exception to be injected.  Maybe it's better to gate the
>> EMULTYPE_SKIP emulation on the exit qualification saying this is a write
> 
> I thought it's already limited to writes. I agree that's a reasonable
> limitation in any case.
> 
>> and also not a page table walk---just in case.
> 
> I still don't get it, sorry. Let's assume for the sake of argument
> that it's a PT walk causing the MMIO access. Just why do you think
> that it makes sense to skip the instruction that caused the walk?

I think it doesn't.  I think in that case it's better to skip the fast
write and proceed with full emulation.

>>> It's just that this has been there for 3 years and people have built a
>>> product around this.
>>
>> Around 700 clock cycles?
> 
> About 30% the cost of exit, isn't it?  There are definitely workloads
> where cost of exit gates performance. We didn't work on fast mmio based
> on theoretical assumptions. But maybe I am wrong. We'll see. Jason here
> volunteered to test your patch and we'll see what comes out of it. If
> I'm wrong and it's about 1%, I won't split hairs.

Note that we still get the latency benefit from fast MMIO, and maybe we
can cut a couple hundred clock cycles more---which would benefit all
emulation, not just fast MMIO.

Paolo