Re: [PATCH] x86/cpuid: expose AVX512_4VNNIW and AVX512_4FMAPS features to kvm guest

[Paolo Bonzini]

On 28/10/2016 11:46, He Chen wrote:
> On Fri, Oct 28, 2016 at 11:31:05AM +0200, Paolo Bonzini wrote:
>>
>>
>> On 28/10/2016 11:12, He Chen wrote:
>>> The spec can be found in Intel Software Developer Manual or in
>>> Instruction Set Extensions Programming Reference.
>>>
>>> Signed-off-by: Luwei Kang <luwei.kang@xxxxxxxxx>
>>> Signed-off-by: He Chen <he.chen@xxxxxxxxxxxxxxx>
>>> ---
>>>  arch/x86/kvm/cpuid.c | 7 ++++++-
>>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>> index afa7bbb..328b169 100644
>>> --- a/arch/x86/kvm/cpuid.c
>>> +++ b/arch/x86/kvm/cpuid.c
>>> @@ -376,6 +376,10 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>>>  	/* cpuid 7.0.ecx*/
>>>  	const u32 kvm_cpuid_7_0_ecx_x86_features = F(PKU) | 0 /*OSPKE*/;
>>>  
>>> +	/* cpuid 7.0.edx*/
>>> +	const u32 kvm_cpuid_7_0_edx_x86_features =
>>> +        0x4 /* AVX512-4VNNIW */ | 0x8 /* AVX512-4FMAPS */;
>>
>> Please define the new features in cpufeature.h first.
>>
> These 2 new features defined as scattered features in kernel.
> In cpufeature.h, there are:
> #define X86_FEATURE_AVX512_4VNNIW (7*32+16)
> #define X86_FEATURE_AVX512_4FMAPS (7*32+17)
> 
> Please see disscusion here:
> https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1250183.html

Uff, that sucks. :(  I'd agree with hpa's position in that thread.

Please do something like

	/* These are scattered features in cpufeature.h.  */
	#define KVM_CPUID_BIT_AVX512_4VNNIW	2
	#define KVM_CPUID_BIT_AVX512_4FMAPS	3
	#define KF(x)	bit(KVM_CPUID_BIT_##x)

and then

	const u32 kvm_cpuid_7_0_edx_x86_features =
		KF(AVX512_4VNNIW) | KF(AVX512_4FMAPS)

I'll think of a trick to avoid using F for scattered features...

Thanks,

Paolo