Subject: Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast
 MMIO
To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
References: <1502890494-35208-1-git-send-email-pbonzini@redhat.com>
 <20170816170843-mutt-send-email-mst@kernel.org>
 <20170816165625.GA32542@flask>
Cc: Paolo Bonzini <pbonzini@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, stable@vger.kernel.org,
        Jason Wang <jasowang@redhat.com>
From: Yang Zhang <yang.zhang.wz@gmail.com>
Message-ID: <1649ef9d-1d78-235d-bb14-ca1949ffc0e9@gmail.com>
Date: Thu, 17 Aug 2017 16:07:17 +0800
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101
 Thunderbird/45.8.0
MIME-Version: 1.0
In-Reply-To: <20170816165625.GA32542@flask>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2498
Lines: 56

On 2017/8/17 0:56, Radim Krčmář wrote:
> 2017-08-16 17:10+0300, Michael S. Tsirkin:
>> On Wed, Aug 16, 2017 at 03:34:54PM +0200, Paolo Bonzini wrote:
>>> 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 way
>>> to keep the full speedup.
>>>
>>> What we can do is use EMULTYPE_SKIP; it only saves 200 clock cycles
>>> because computing the physical RIP and reading the instruction is
>>> expensive, but at least the eventfd is signaled before entering the
>>> emulator.  This saves on latency.  While at it, don't check breakpoints
>>> when skipping the instruction, as presumably any side effect has been
>>> exposed already.
>>>
>>> Adding a hypercall or MSR write that does a fast MMIO write to a physical
>>> address would do it, but it adds hypervisor knowledge in virtio, including
>>> CPUID handling.  So it would be pretty ugly in the guest-side implementation,
>>> but if somebody wants to do it and the virtio side is acceptable to the
>>> virtio maintainers, I am okay with it.
>>>
>>> Cc: Michael S. Tsirkin <mst@redhat.com>
>>> Cc: stable@vger.kernel.org
>>> Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae
>>> Suggested-by: Radim Krčmář <rkrcmar@redhat.com>
>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>
>> Jason (cc) who worked on the original optimization said he can
>> work to test the performance impact.
>> I suggest we don't rush this (it's been like this for 2 years),
>> and the issue seems to be largely theoretical.
>
> Paolo, did Microsoft point it out because they hit the bug when running
> KVM on Hyper-V?

Does this mean the nested emulation of EPT violation and 
misconfiguration in KVM side doesn't strictly follow the manual since we 
didn't hit the bug in KVM?

>
> Thanks.
>


-- 
Yang
Alibaba Cloud Computing