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

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Message-ID: <1530586086.975916.1502893030374.JavaMail.zimbra@redhat.com>
Date:   Wed, 16 Aug 2017 10:17:10 -0400 (EDT)
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


> We actually know what to expect (a write) so we could maybe
> optimize this some more with a dedicated function just for this.

We don't know the addressing mode, the size or the source (immediate
vs. register), so no.

KVM is already doing a single translation and read no matter how long
the instruction (unless it cross page boundaries).

Paolo

> > 
> > However, the eventfd is written before decoding, while full emulation
> > would write it after. So while VCPU thread latency is worse compared to
> > skip_emulated_instruction, latency to the iothread remains small.
> > 
> > Paolo
>