Re: [PATCH] kvm: x86: emulate MSR_PLATFORM_INFO msr bits

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 8/22/2023 12:11 AM, Sean Christopherson wrote:
> +Aaron
> 
> When resending a patch, e.g. to change To: or Cc:, tag it RESEND.  I got three
> copies of this...
> 
> On Mon, Aug 21, 2023, Hao Xiang wrote:
>>
>>
>> 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.
> 
> It doesn't conflict per se, but it's still problematic.  By stuffing a default
> value, KVM _forces_ userspace to override the MSR to align with the topology and
> CPUID defined by userspace.  

I don't understand how this MSR is related to topology and CPUID?

> And if userspace uses KVM's "default" CPUID, or lack
> thereof, using the underlying values from hardware are all but guaranteed to be
> wrong.

Could you please elaborate?

> The existing code that sets MSR_PLATFORM_INFO_CPUID_FAULT really should not exist,
> i.e. KVM shouldn't shouldn't assume userspace wants to expose CPUID faulting to
> the guest.  That particular one probably isn't worth trying to retroactively fix.
> 
> Ditto for setting MSR_IA32_ARCH_CAPABILITIES; KVM is overstepping, but doing so
> likely doesn't cause problems.
> 
> MSR_IA32_PERF_CAPABILITIES is a different story.  Setting a non-zero default value
> is blatantly wrong, as KVM will advertise vPMU features even if userspace doesn't
> advertise.  Aaron is planning on sending a patch for this one (I'm hoping we can
> get away with retroactively dropping the code without having to add a quirk).
> 
> *If* we need KVM to expose the ratios to userspace, then the correct way to do so
> is handle turbo and efficiency ratio information is to by implementing support in
> kvm_get_msr_feature(), i.e. KVM_GET_MSRS on /dev/kvm.  Emphasis on "if", because
> I would prefer to do nothing in KVM if that information is already surfaced to
> userspace through other mechanisms in the kernel.