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

[David Woodhouse <dwmw2@infradead.org>]

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On Fri, 2018-02-02 at 14:56 -0500, Konrad Rzeszutek Wilk wrote:
> On Fri, Feb 02, 2018 at 06:02:24PM +0000, David Woodhouse wrote:
> > 
> > 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,
> > > >                  if (!g_phys_as)
> > > >                          g_phys_as = phys_as;
> > > >                  entry->eax = g_phys_as | (virt_as << 8);
> > > > -               entry->ebx = entry->edx = 0;
> > > > +               entry->edx = 0;
> > > > +               /* IBPB isn't necessarily present in hardware cpuid>  */
> > > > +               if (boot_cpu_has(X86_FEATURE_IBPB))
> > > > +                       entry->ebx |= F(IBPB);
> > > > +               entry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
> > > > +               cpuid_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.
>
> But this seems wrong. That is on Intel CPUs we will advertise on
> AMD leaf that the IBPB feature is available.
> 
> Shouldn't we just check to see if the machine is AMD before advertising
> this bit?

No. The AMD feature bits give us more fine-grained support for exposing
IBPB or IBRS alone, so we expose those bits on Intel too.

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