linux-kernel - Re: [PATCH] kvm: x86: emulate MSR_PLATFORM

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Message-ID: <33f0e9bb-da79-6f32-f1c3-816eb37daea6@linux.alibaba.com>
Date:   Mon, 21 Aug 2023 19:44:09 +0800
From:   Hao Xiang <hao.xiang@...ux.alibaba.com>
To:     Chao Gao <chao.gao@...el.com>
Cc:     kvm@...r.kernel.org, shannon.zhao@...ux.alibaba.com,
        pbonzini@...hat.com, seanjc@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: emulate MSR_PLATFORM_INFO msr bits



On 2023/8/21 18:44, Chao Gao wrote:
> On Mon, Aug 21, 2023 at 05:11:16PM +0800, Hao Xiang wrote:
>> For reason that,
>>
>> The turbo frequency info depends on specific machine type. And the msr value
>> of MSR_PLATFORM_INFO may be diferent on diffrent generation machine.
>>
>> Get following msr bits (needed by turbostat on intel platform) by rdmsr
>> MSR_PLATFORM_INFO directly in KVM is more reasonable. And set these msr bits
>> as vcpu->arch.msr_platform_info default value.
>> -bit 15:8, Maximum Non-Turbo Ratio (MAX_NON_TURBO_LIM_RATIO)
>> -bit 47:40, Maximum Efficiency Ratio (MAX_EFFICIENCY_RATIO)
> 
> I don't get why QEMU cannot do this with the existing interface, e.g.,
> KVM_SET_MSRS.
> 
> will the MSR value be migrated during VM migration?
> 
> looks we are in a dilemma. on one side, if the value is migrated, the value can
> become inconsisntent with hardware value. On the other side, changing the ratio
> bits at runtime isn't the architectural behavior.
> 
> And the MSR is per-socket. In theory, a system can have two sockets with
> different values of the MSR. what if a vCPU is created on a socket and then
> later runs on the other socket?
> 

Set these msr bits (needed by turbostat on intel platform) in KVM by 
default.
Of cource, QEMU can also set MSR value by need. It does not conflict.

>>
>> On 2023/8/21 15:52, Chao Gao wrote:
>>> On Mon, Aug 21, 2023 at 11:26:32AM +0800, Hao Xiang wrote:
>>>> For intel platform, The BzyMhz field of Turbostat shows zero
>>>> due to the missing of part msr bits of MSR_PLATFORM_INFO.
>>>>
>>>> Acquire necessary msr bits, and expose following msr info to guest,
>>>> to make sure guest can get correct turbo frequency info.
>>>
>>> Userspace VMM (e.g., QEMU) can configure this MSR for guests. Please refer to
>>> tools/testing/selftests/kvm/x86_64/platform_info_test.c.
>>>
>>> The question is why KVM needs this patch given KVM already provides interfaces
>>> for QEMU to configure the MSR.