Re: [Xen-devel] [PATCH 2/6] x86emul: support WBNOINVD

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.08.2019 16:47, Andrew Cooper wrote:
> On 01/07/2019 12:56, Jan Beulich wrote:
> > --- a/xen/arch/x86/pv/emul-priv-op.c
> > +++ b/xen/arch/x86/pv/emul-priv-op.c
> > @@ -1121,7 +1121,7 @@ static int write_msr(unsigned int reg, u
> > @@ -1130,6 +1130,8 @@ static int cache_op(enum x86emul_cache_o
> >             * newer linux uses this in some start-of-day timing loops.
> >             */
> >            ;
> > +    else if ( op == x86emul_wbnoinvd && cpu_has_wbnoinvd )
> > +        wbnoinvd();
> >        else
> >            wbinvd();
> The cpu_has_wbnoinvd check isn't necessary.  The encoding was chosen
> because it does get interpreted as wbinvd on older processors.
I agree, but wanted to make the code look complete / consistent.
Would you be okay with the && ... being retained, but in a
comment?

> > --- a/xen/include/asm-x86/system.h
> > +++ b/xen/include/asm-x86/system.h
> > @@ -16,6 +16,11 @@ static inline void wbinvd(void)
> >        asm volatile ( "wbinvd" ::: "memory" );
> >    }
> >    
> > +static inline void wbnoinvd(void)
> > +{
> > +    asm volatile ( "repe; wbinvd" : : : "memory" );
> Semicolon.
It has to stay, as gas rejects use of REP on insns it doesn't think
permit use of REP. H.J. actually proposes even more strict (or should
I say hostile) gas behavior, which would then also reject the above
construct:
https://sourceware.org/ml/binutils/2019-07/msg00186.html

> > --- a/xen/include/public/arch-x86/cpufeatureset.h
> > +++ b/xen/include/public/arch-x86/cpufeatureset.h
> > @@ -243,6 +244,7 @@ XEN_CPUFEATURE(EFRO,          7*32+10) /
> >    
> >    /* AMD-defined CPU features, CPUID level 0x80000008.ebx, word 8 */
> >    XEN_CPUFEATURE(CLZERO,        8*32+ 0) /*A  CLZERO instruction */
> > +XEN_CPUFEATURE(WBNOINVD,      8*32+ 9) /*A  WBNOINVD instruction */
> This is implicitly linked with CPUID.8000001d which we don't expose yet.
On AMD, but not (so far at least, judging from the SDM) on Intel.

> To get the emulation side of things sorted, I'd be happy with this going
> in without the A for now, and "exposing WBNOINVD to guests" can be a
> followup task.
I've dropped the A for now, but as per above I'm not entirely
certain that's appropriate; it's certainly the more defensive step.
My uncertainty is also because people are free to use the WBNOINVD
encoding even without the feature flag set, as it won't #UD (as you
also suggest elsewhere in your reply).

Jan

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