Re: [PATCH 6/6] KVM: x86: Set KVM LASS based on hardware capability

[Zeng Guang]

On 4/25/2023 10:57 AM, Binbin Wu wrote:
> On 4/20/2023 9:37 PM, Zeng Guang wrote:
> > Host kernel may clear LASS capability in boot_cpu_data.x86_capability
> Is there some option to do it?

Kernel supporting LASS will turn off the LASS capability with specific 
option, e.g.
"vsyscall=emulate".

> > besides explicitly using clearcpuid parameter. That will cause guest
> > not being able to manage LASS independently. So set KVM LASS directly
> > based on hardware capability to eliminate the dependency.
> >
> > Add new helper functions to facilitate getting result of CPUID sub-leaf.
> >
> > Signed-off-by: Zeng Guang <guang.zeng@xxxxxxxxx>
> > ---
> >    arch/x86/include/asm/cpuid.h | 36 ++++++++++++++++++++++++++++++++++++
> >    arch/x86/kvm/cpuid.c         |  4 ++++
> >    2 files changed, 40 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/cpuid.h b/arch/x86/include/asm/cpuid.h
> > index 9bee3e7bf973..a25dd00b7c0a 100644
> > --- a/arch/x86/include/asm/cpuid.h
> > +++ b/arch/x86/include/asm/cpuid.h
> > @@ -127,6 +127,42 @@ static inline unsigned int cpuid_edx(unsigned int op)
> >    	return edx;
> >    }
> >    
> > +static inline unsigned int cpuid_count_eax(unsigned int op, int count)
> > +{
> > +	unsigned int eax, ebx, ecx, edx;
> > +
> > +	cpuid_count(op, count, &eax, &ebx, &ecx, &edx);
> > +
> > +	return eax;
> > +}
> > +
> > +static inline unsigned int cpuid_count_ebx(unsigned int op, int count)
> > +{
> > +	unsigned int eax, ebx, ecx, edx;
> > +
> > +	cpuid_count(op, count, &eax, &ebx, &ecx, &edx);
> > +
> > +	return ebx;
> > +}
> > +
> > +static inline unsigned int cpuid_count_ecx(unsigned int op, int count)
> > +{
> > +	unsigned int eax, ebx, ecx, edx;
> > +
> > +	cpuid_count(op, count, &eax, &ebx, &ecx, &edx);
> > +
> > +	return ecx;
> > +}
> > +
> > +static inline unsigned int cpuid_count_edx(unsigned int op, int count)
> > +{
> > +	unsigned int eax, ebx, ecx, edx;
> > +
> > +	cpuid_count(op, count, &eax, &ebx, &ecx, &edx);
> > +
> > +	return edx;
> > +}
> > +
> >    static __always_inline bool cpuid_function_is_indexed(u32 function)
> >    {
> >    	switch (function) {
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 5facb8037140..e99b99ebe1fe 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -667,6 +667,10 @@ void kvm_set_cpu_caps(void)
> >    		F(AMX_FP16) | F(AVX_IFMA)
> >    	);
> >    
> > +	/* Set LASS based on hardware capability */
> > +	if (cpuid_count_eax(7, 1) & F(LASS))
> > +		kvm_cpu_cap_set(X86_FEATURE_LASS);
> > +
> >    	kvm_cpu_cap_init_kvm_defined(CPUID_7_1_EDX,
> >    		F(AVX_VNNI_INT8) | F(AVX_NE_CONVERT) | F(PREFETCHITI)
> >    	);