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

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Message-ID: <0554a813-f1a4-f350-005f-6a7d0c54b92a@redhat.com>
Date:   Thu, 17 Aug 2017 10:15:02 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        "Michael S . Tsirkin" <mst@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS

On 16.08.2017 14:07, Radim Krčmář wrote:
> 2017-08-16 13:22+0200, Paolo Bonzini:
>> Microsoft pointed out privately to me that KVM's handling of
>> KVM_FAST_MMIO_BUS is invalid.  Using skip_emulation_instruction is invalid
>> in EPT misconfiguration vmexit handlers, because neither EPT violations
>> nor misconfigurations are listed in the manual among the VM exits that
>> set the VM-exit instruction length field.
>>
>> While physical processors seem to set the field, this is not architectural
>> and is just a side effect of the implementation.  I couldn't convince
>> myself of any condition on the exit qualification where VM-exit
>> instruction length "has" to be defined; there are no trap-like VM-exits
>> that can be repurposed; and fault-like VM-exits such as descriptor-table
>> exits provide no decoding information.  So I don't really see any elegant
>> way to fix it except by disabling KVM_FAST_MMIO_BUS, which means virtio
>> 1 will go slower.
> 
> Do you have some numbers?
> 
> We could keep the ugliness in KVM and add a new skip function with
> emulate_instruction(vcpu, EMULTYPE_SKIP) to decode the length of the
> instruction.  (Adding a condition just for EPT violation exit reason to

I like that idea.

> the existing skip function would be a dirtier solution.)
> Slower than what we have now, but faster than full emulation.
> 
> I agree that configuring EPT to throw a violation when accessing fast
> MMIO has many drawbacks (although it seems to be what Intel expected).
> 
> Thanks.


-- 

Thanks,

David