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

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Message-ID: <20201019184849.GC3713473@iweiny-DESK2.sc.intel.com>
Date:   Mon, 19 Oct 2020 11:48:49 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        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>,
        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 V3 4/9] x86/pks: Preserve the PKRS MSR on context
 switch

On Mon, Oct 19, 2020 at 11:37:14AM +0200, Peter Zijlstra wrote:
> On Fri, Oct 16, 2020 at 10:14:10PM -0700, Ira Weiny wrote:
> > > 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? 
> 
> It does.
> 
> > Is it not sufficient to rely on that?
> 
> It is.
> 
> > 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().
> 
> Dave's concern is that we're calling with with preemption already
> disabled so disabling it again is superfluous.

Ok, thanks, and after getting my head straight I think I agree with him, and
you.

Thanks I've reworked the code to removed the superfluous calls.  Sorry about
being so dense...  :-D

Ira