Re: [PATCH RFC V3 4/9] x86/pks: Preserve the PKRS MSR on context switch

[Ira Weiny]

On Fri, Oct 16, 2020 at 01:06:36PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 09, 2020 at 12:42:53PM -0700, ira.weiny@xxxxxxxxx wrote:
> 
> > @@ -644,6 +663,8 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)
> >  
> >  	if ((tifp ^ tifn) & _TIF_SLD)
> >  		switch_to_sld(tifn);
> > +
> > +	pks_sched_in();
> >  }
> >  
> 
> You seem to have lost the comment proposed here:
> 
>   https://lkml.kernel.org/r/20200717083140.GW10769@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 
> It is useful and important information that the wrmsr normally doesn't
> happen.

Added back in here.

> 
> > diff --git a/arch/x86/mm/pkeys.c b/arch/x86/mm/pkeys.c
> > index 3cf8f775f36d..30f65dd3d0c5 100644
> > --- a/arch/x86/mm/pkeys.c
> > +++ b/arch/x86/mm/pkeys.c
> > @@ -229,3 +229,31 @@ u32 update_pkey_val(u32 pk_reg, int pkey, unsigned int flags)
> >  
> >  	return pk_reg;
> >  }
> > +
> > +DEFINE_PER_CPU(u32, pkrs_cache);
> > +
> > +/**
> > + * It should also be noted that the underlying WRMSR(MSR_IA32_PKRS) is not
> > + * serializing but still maintains ordering properties similar to WRPKRU.
> > + * The current SDM section on PKRS needs updating but should be the same as
> > + * that of WRPKRU.  So to quote from the WRPKRU text:
> > + *
> > + * 	WRPKRU will never execute transiently. Memory accesses
> > + * 	affected by PKRU register will not execute (even transiently)
> > + * 	until all prior executions of WRPKRU have completed execution
> > + * 	and updated the PKRU register.
> 
> (whitespace damage; space followed by tabstop)

Fixed thanks.

> 
> > + */
> > +void write_pkrs(u32 new_pkrs)
> > +{
> > +	u32 *pkrs;
> > +
> > +	if (!static_cpu_has(X86_FEATURE_PKS))
> > +		return;
> > +
> > +	pkrs = get_cpu_ptr(&pkrs_cache);
> > +	if (*pkrs != new_pkrs) {
> > +		*pkrs = new_pkrs;
> > +		wrmsrl(MSR_IA32_PKRS, new_pkrs);
> > +	}
> > +	put_cpu_ptr(pkrs);
> > +}
> 
> looks familiar that... :-)

Added you as a co-developer if that is ok?

Ira