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SubjectRe: [PATCH RFC V3 4/9] x86/pks: Preserve the PKRS MSR on context switch
On Fri, Oct 16, 2020 at 01:12:26PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 13, 2020 at 11:31:45AM -0700, Dave Hansen wrote:
> > > +/**
> > > + * 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.
> > > + */
> > > +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);
> > > +}
> > >
> >
> > It bugs me a *bit* that this is being called in a preempt-disabled
> > region, but we still bother with the get/put_cpu jazz. Are there other
> > future call-sites for this that aren't in preempt-disabled regions?
>
> So the previous version had a useful comment that got lost.

Ok Looking back I see what happened... This comment...

/*
* PKRS is only temporarily changed during specific code paths.
* Only a preemption during these windows away from the default
* value would require updating the MSR.
*/

... was added to pks_sched_in() but that got simplified down because cleaning
up write_pkrs() made the code there obsolete.

> This stuff
> needs to fundamentally be preempt disabled,

I agree, the update to the percpu cache value and MSR can not be torn.

> so it either needs to
> explicitly do so, or have an assertion that preemption is indeed
> disabled.

However, I don't think I understand clearly. Doesn't [get|put]_cpu_ptr()
handle the preempt_disable() for us? Is it not sufficient to rely on that?

Dave's comment seems to be the opposite where we need to eliminate preempt
disable before calling write_pkrs().

FWIW I think I'm mistaken in my response to Dave regarding the
preempt_disable() in pks_update_protection().

Ira

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