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Date: Wed, 16 Aug 2017 10:17:10 -0400 (EDT)
From: Paolo Bonzini <pbonzini@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        stable@vger.kernel.org
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Subject: Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS
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> 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
>