linux-kernel - Re: [PATCH] KVM: x86/cpuid: Clear XFD for component i if the base feature is missing

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Message-ID: <5ca97b8f-facd-b1dc-f671-51569ad17284@gmail.com>
Date:   Tue, 18 Jan 2022 14:43:51 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Jing Liu <jing2.liu@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/cpuid: Clear XFD for component i if the base
 feature is missing

On 18/1/2022 1:31 am, Paolo Bonzini wrote:
> On 1/17/22 08:45, Like Xu wrote:
>> From: Like Xu <likexu@...cent.com>
>>
>> According to Intel extended feature disable (XFD) spec, the sub-function i
>> (i > 1) of CPUID function 0DH enumerates "details for state component i.
>> ECX[2] enumerates support for XFD support for this state component."
>>
>> If KVM does not report F(XFD) feature (e.g. due to CONFIG_X86_64),
>> then the corresponding XFD support for any state component i
>> should also be removed. Translate this dependency into KVM terms.
>>
>> Fixes: 690a757d610e ("kvm: x86: Add CPUID support for Intel AMX")
>> Signed-off-by: Like Xu <likexu@...cent.com>
>> ---
>>   arch/x86/kvm/cpuid.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index c55e57b30e81..e96efef4f048 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -886,6 +886,9 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array 
>> *array, u32 function)
>>                   --array->nent;
>>                   continue;
>>               }
>> +
>> +            if (!kvm_cpu_cap_has(X86_FEATURE_XFD))
>> +                entry->ecx &= ~BIT_ULL(2);
>>               entry->edx = 0;
>>           }
>>           break;
> 
> Generally this is something that is left to userspace.  Apart from the usecase 
> of "call KVM_GET_SUPPORTED_CPUID and pass it to KVM_SET_CPUID2", userspace 
> should know what any changed bits mean.
> 
> Paolo
> 

I totally agree that setting the appropriate CPUID bits for a feature is a user 
space thing.

But this patch is more focused on fixing a different type of problem, which is
that the capabilities exposed by KVM should not *contradict each other* :

	a user space may be confused with the current code base :
	- why KVM does not have F(XFD) feature (MSR_IA32_XFD and XFD_ERR non-exit),
	- but KVM reports XFD support for state component i individually;

This is like KVM reporting PEBS when no PMU capacity is reported (due to module 
param).

Does this clarification help ?

Thanks,
Like Xu