Re: [PATCH v2 22/49] KVM: x86: Add a macro to precisely handle aliased 0x1.EDX CPUID features

[Sean Christopherson]

On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote:
> > Add a macro to precisely handle CPUID features that AMD duplicated from
> > CPUID.0x1.EDX into CPUID.0x8000_0001.EDX.  This will allow adding an
> > assert that all features passed to kvm_cpu_cap_init() match the word being
> > processed, e.g. to prevent passing a feature from CPUID 0x7 to CPUID 0x1.
> > 
> > Because the kernel simply reuses the X86_FEATURE_* definitions from
> > CPUID.0x1.EDX, KVM's use of the aliased features would result in false
> > positives from such an assert.
> > 
> > No functional change intended.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/cpuid.c | 24 +++++++++++++++++-------
> >  1 file changed, 17 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 5e3b97d06374..f2bd2f5c4ea3 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -88,6 +88,16 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
> >  	F(name);						\
> >  })
> >  
> > +/*
> > + * Aliased Features - For features in 0x8000_0001.EDX that are duplicates of
> > + * identical 0x1.EDX features, and thus are aliased from 0x1 to 0x8000_0001.
> > + */
> > +#define AF(name)								\
> > +({										\
> > +	BUILD_BUG_ON(__feature_leaf(X86_FEATURE_##name) != CPUID_1_EDX);	\
> > +	feature_bit(name);							\
> > +})
> > +
> >  /*
> >   * Magic value used by KVM when querying userspace-provided CPUID entries and
> >   * doesn't care about the CPIUD index because the index of the function in
> > @@ -758,13 +768,13 @@ void kvm_set_cpu_caps(void)
> >  	);
> >  
> >  	kvm_cpu_cap_init(CPUID_8000_0001_EDX,
> > -		F(FPU) | F(VME) | F(DE) | F(PSE) |
> > -		F(TSC) | F(MSR) | F(PAE) | F(MCE) |
> > -		F(CX8) | F(APIC) | 0 /* Reserved */ | F(SYSCALL) |
> > -		F(MTRR) | F(PGE) | F(MCA) | F(CMOV) |
> > -		F(PAT) | F(PSE36) | 0 /* Reserved */ |
> > -		F(NX) | 0 /* Reserved */ | F(MMXEXT) | F(MMX) |
> > -		F(FXSR) | F(FXSR_OPT) | X86_64_F(GBPAGES) | F(RDTSCP) |
> > +		AF(FPU) | AF(VME) | AF(DE) | AF(PSE) |
> > +		AF(TSC) | AF(MSR) | AF(PAE) | AF(MCE) |
> > +		AF(CX8) | AF(APIC) | 0 /* Reserved */ | F(SYSCALL) |
> > +		AF(MTRR) | AF(PGE) | AF(MCA) | AF(CMOV) |
> > +		AF(PAT) | AF(PSE36) | 0 /* Reserved */ |
> > +		F(NX) | 0 /* Reserved */ | F(MMXEXT) | AF(MMX) |
> > +		AF(FXSR) | F(FXSR_OPT) | X86_64_F(GBPAGES) | F(RDTSCP) |
> >  		0 /* Reserved */ | X86_64_F(LM) | F(3DNOWEXT) | F(3DNOW)
> >  	);
> >  
> 
> Hi,
> 
> What if we defined the aliased features instead.
> Something like this:
> 
> #define __X86_FEATURE_8000_0001_ALIAS(feature) \
> 	(feature + (CPUID_8000_0001_EDX - CPUID_1_EDX) * 32)
> 
> #define KVM_X86_FEATURE_FPU_ALIAS	__X86_FEATURE_8000_0001_ALIAS(KVM_X86_FEATURE_FPU)
> #define KVM_X86_FEATURE_VME_ALIAS	__X86_FEATURE_8000_0001_ALIAS(KVM_X86_FEATURE_VME)
> 
> And then just use for example the 'F(FPU_ALIAS)' in the CPUID_8000_0001_EDX

At first glance, I really liked this idea, but after working through the
ramifications, I think I prefer "converting" the flag when passing it to
kvm_cpu_cap_init().  In-place conversion makes it all but impossible for KVM to
check the alias, e.g. via guest_cpu_cap_has(), especially since the AF() macro
doesn't set the bits in kvm_known_cpu_caps (if/when a non-hacky validation of
usage becomes reality).

Side topic, if it's not already documented somewhere else, kvm/x86/cpuid.rst
should call out that KVM only honors the features in CPUID.0x1, i.e. that setting
aliased bits in CPUID.0x8000_0001 is supported if and only if the bit(s) is also
set in CPUID.0x1.