Re: [PATCH v2 2/2] KVM: x86: Expose Predictive Store Forwarding Disable on Intel parts

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 30, 2022, Jim Mattson wrote:
> On Tue, Aug 30, 2022 at 4:39 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Tue, Aug 30, 2022, Jim Mattson wrote:
> > > Intel enumerates support for PSFD in CPUID.(EAX=7,ECX=2):EDX.PSFD[bit
> > > 0]. Report support for this feature in KVM if it is available on the
> > > host.
> > >
> > > Presumably, this feature bit, like its AMD counterpart, is not welcome
> > > in cpufeatures.h, so add a local definition of this feature in KVM.
> > >
> > > Signed-off-by: Jim Mattson <jmattson@google.com>
> > > ---
> > >  arch/x86/kvm/cpuid.c | 23 +++++++++++++++++------
> > >  1 file changed, 17 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index 07be45c5bb93..b5af9e451bef 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -62,6 +62,7 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
> > >   * This one is tied to SSB in the user API, and not
> > >   * visible in /proc/cpuinfo.
> > >   */
> > > +#define KVM_X86_FEATURE_PSFD         0          /* Predictive Store Forwarding Disable */
> >
> > I believe we can use "enum kvm_only_cpuid_leafs" to handle this.  E.g.
> >
> >         enum kvm_only_cpuid_leafs {
> >                 CPUID_12_EAX     = NCAPINTS,
> >                 CPUID_7_2_EDX,
> >                 NR_KVM_CPU_CAPS,
> >
> >                 NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
> >         };
> >
> > then the intended use of KVM_X86_FEATURE_*
> >
> >         #define KVM_X86_FEATURE_PSFD    KVM_X86_FEATURE(CPUID_7_2_EDX, 0)
> >
> > I _think_ we can then define an arbitrary word for X86_FEATURE_PSFD, e.g.
> >
> >         #define X86_FEATURE_PSFD        (NKVMCAPINTS*32+0)
> 
> We may run afoul of reverse_cpuid_check(), depending on usage.

Oh, yeah, an entry in reverse_cpuid[] would also be needed.  For posterity since
PSFD doesn't need a KVM-only entry[*]...

[*] https://lore.kernel.org/all/Y0CrER%2FyiHheWiZw@google.com