Re: [PATCH RFC V2 04/17] x86/pks: Preserve the PKRS MSR on context switch

Peter Zijlstra
2020-07-20T09:16:05Z

On Fri, Jul 17, 2020 at 03:34:07PM -0700, Ira Weiny wrote:
> On Fri, Jul 17, 2020 at 10:59:54AM +0200, Peter Zijlstra wrote:
> > On Fri, Jul 17, 2020 at 12:20:43AM -0700, ira.weiny@intel.com wrote:
> > > +/*
> > > + * Write the PKey Register Supervisor. This must be run with preemption
> > > + * disabled as it does not guarantee the atomicity of updating the pkrs_cache
> > > + * and MSR on its own.
> > > + */
> > > +void write_pkrs(u32 pkrs_val)
> > > +{
> > > + this_cpu_write(pkrs_cache, pkrs_val);
> > > + wrmsrl(MSR_IA32_PKRS, pkrs_val);
> > > +}
> >
> > Should we write that like:
> >
> > void write_pkrs(u32 pkr)
> > {
> > u32 *pkrs = get_cpu_ptr(pkrs_cache);
> > if (*pkrs != pkr) {
> > *pkrs = pkr;
> > wrmsrl(MSR_IA32_PKRS, pkr);
> > }
> > put_cpu_ptrpkrs_cache);
> > }
> >
> > given that we fundamentally need to serialize againt schedule() here.
>
> Yes. That seems better.
>
> That also means pks_sched_in() can be simplified to just
>
> static inline void pks_sched_in(void)
> {
> write_pkrs(current->thread.saved_pkrs);
> }
>
> Because of the built WRMSR avoidance.
>
> However, pkrs_cache is static so I think I need to use {get,put}_cpu_var() here
> don't I?

Or get_cpu_ptr(&pkrs_cache), sorry for the typo :-)