Re: [PATCH v6 2/5] KVM: x86: Add IBPB support

[David Woodhouse]

On Fri, 2018-02-02 at 12:49 -0500, Konrad Rzeszutek Wilk wrote:
> > @@ -625,7 +629,12 @@ static inline int __do_cpuid_ent(struct
> > kvm_cpuid_entry2 *entry, u32 function,
> >ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂif (!g_phys_as)
> >ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂg_phys_as = phys_as;
> >ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂentry->eax = g_phys_as | (virt_as << 8);
> > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂentry->ebx = entry->edx = 0;
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂentry->edx = 0;
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ/* IBPB isn't necessarily present in hardware cpuid>  */
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂif (boot_cpu_has(X86_FEATURE_IBPB))
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂentry->ebx |= F(IBPB);
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂentry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂcpuid_mask(&entry->ebx, CPUID_8000_0008_EBX);
> 
> It is with x86/pti nowadays. I think you can remove that comment.

In this code we use the actual CPUID instruction, then filter stuff out
of it (with &= kvm_cpuid_XXX_x86_features and then cpuid_mask() to turn
off any bits which were otherwise present in the hardware and *would*
have been supported by KVM, but which the kernel has decided to pretend
are not present.

Nothing would *set* the IBPB bit though, since that's a "virtual" bit
on Intel hardware. The comment explains why we have that |= F(IBPB),
and if the comment wasn't true, we wouldn't need that code either.

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