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

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Message-ID: <2f80e82b-3007-0348-72b8-3391c7c57589@redhat.com>
Date:   Thu, 17 Aug 2017 14:14:59 +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 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.

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

Paolo