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

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Message-ID: <20170817132354.GB2566@flask>
Date:   Thu, 17 Aug 2017 15:23:55 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...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

2017-08-17 14:14+0200, Paolo Bonzini:
> On 17/08/2017 11:00, Paolo Bonzini wrote:
> > 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 definitely doesn't.  I think it's possible only because virtio
forbids other access types, so we assume that every access is a write
access.

> ... nope, exit qualification is just zero for EPT misconfigurations, so
> we cannot do this.

Yeah, we'd need to use EPT violation to get that information.