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

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Message-ID: <20200717223407.GS3008823@iweiny-DESK2.sc.intel.com>
Date:   Fri, 17 Jul 2020 15:34:07 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC V2 04/17] x86/pks: Preserve the PKRS MSR on context
 switch

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@...el.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?

Thanks!
Ira