Re: [PATCH v2 24/49] KVM: x86: #undef SPEC_CTRL_SSBD in cpuid.c to avoid macro collisions

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
> > Undefine SPEC_CTRL_SSBD, which is #defined by msr-index.h to represent the
> > enable flag in MSR_IA32_SPEC_CTRL, to avoid issues with the macro being
> > unpacked into its raw value when passed to KVM's F() macro.  This will
> > allow using multiple layers of macros in F() and friends, e.g. to harden
> > against incorrect usage of F().
> > 
> > No functional change intended (cpuid.c doesn't consume SPEC_CTRL_SSBD).
> > 
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/kvm/cpuid.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 8efffd48cdf1..a16d6e070c11 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -639,6 +639,12 @@ static __always_inline void kvm_cpu_cap_init(u32 leaf, u32 mask)
> >  	kvm_cpu_caps[leaf] &= raw_cpuid_get(cpuid);
> >  }
> >  
> > +/*
> > + * Undefine the MSR bit macro to avoid token concatenation issues when
> > + * processing X86_FEATURE_SPEC_CTRL_SSBD.
> > + */
> > +#undef SPEC_CTRL_SSBD
> > +
> >  void kvm_set_cpu_caps(void)
> >  {
> >  	memset(kvm_cpu_caps, 0, sizeof(kvm_cpu_caps));
> 
> Hi,
> 
> Maybe we should instead rename the SPEC_CTRL_SSBD to
> 'MSR_IA32_SPEC_CTRL_SSBD' and together with it, other fields of this msr.  It
> seems that at least some msrs in this file do this.

Yeah, the #undef hack is quite ugly.  But I didn't (and still don't) want to
introduce all the renaming churn in the middle of this already too-big series,
especially since it would require touching quite a bit of code outside of KVM.

I'm also not sure that's the right thing to do; I kinda feel like KVM is the one
that's being silly here.

Aha!  Rather than rename the MSR bits, what if we rename the X86_FEATURE flag,
e.g. to X86_FEATURE_INTEL_SPEC_CTRL_SSBD, X86_FEATURE_MSR_SPEC_CTRL_SSBD, or maybe
even just X86_FEATURE_INTEL_SSBD.  Much less churn, and it would add even more
clarity as to why there's also X86_FEATURE_SSBD and X86_FEATURE_AMD_SSBD.

I'll post a standalone patch to make that change, and maybe see if I can take it
through the KVM tree.