linux-kernel - Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO

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Date:   Thu, 17 Aug 2017 16:31:14 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Yang Zhang <yang.zhang.wz@...il.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, "# v3 . 10+" <stable@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO

2017-08-17 16:28 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> 2017-08-17 16:07 GMT+08:00 Yang Zhang <yang.zhang.wz@...il.com>:
>> 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@...hat.com>
>>>>> Cc: stable@...r.kernel.org
>>>>> Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae
>>>>> Suggested-by: Radim Krčmář <rkrcmar@...hat.com>
>>>>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.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?
>
> The VM-exit instruction length of vmcs12 is provided by vmcs02
> (prepare_vmcs12()), so unless the length from vmcs02 is wrong. In
> addition, something like mov instruction which can trigger the EPT
> violation/misconfig in guest has already been decoded before executing
> I think, IIUC, then exit qualification can have the information about
> the instruction length.

s/exit qualification/VM-exit instruction length