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

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

[Like Xu]

From: Like Xu <likexu@tencent.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@tencent.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;
-- 
2.33.1

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

[Paolo Bonzini]

On 1/17/22 08:45, Like Xu wrote:
> From: Like Xu <likexu@tencent.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@tencent.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

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

[Like Xu]

On 18/1/2022 1:31 am, Paolo Bonzini wrote:
> On 1/17/22 08:45, Like Xu wrote:
>> From: Like Xu <likexu@tencent.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@tencent.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

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

[Paolo Bonzini]

On 1/18/22 07:43, Like Xu wrote:
> On 18/1/2022 1:31 am, Paolo Bonzini wrote:
>> On 1/17/22 08:45, Like Xu wrote:
>>> From: Like Xu <likexu@tencent.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@tencent.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;

Got it.  Yeah, the patch makes sense for the sake of CONFIG_X86_64.

Paolo

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