linux-kernel - Re: [RFC PATCH 04/15] x86/pks: Preserve the PKRS MSR on context switch

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Date:   Tue, 14 Jul 2020 21:05:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ira Weiny <ira.weiny@...el.com>
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: [RFC PATCH 04/15] x86/pks: Preserve the PKRS MSR on context
 switch

On Tue, Jul 14, 2020 at 11:53:22AM -0700, Ira Weiny wrote:
> On Tue, Jul 14, 2020 at 10:27:01AM +0200, Peter Zijlstra wrote:
> > On Tue, Jul 14, 2020 at 12:02:09AM -0700, ira.weiny@...el.com wrote:
> > > From: Ira Weiny <ira.weiny@...el.com>
> > > 
> > > The PKRS MSR is defined as a per-core register.  This isolates memory
> > > access by CPU.  Unfortunately, the MSR is not preserved by XSAVE.
> > > Therefore, We must preserve the protections for individual tasks even if
> > > they are context switched out and placed on another cpu later.
> > 
> > This is a contradiction and utter trainwreck.
> 
> I don't understand where there is a contradiction?  Perhaps I should have said
> the MSR is not XSAVE managed vs 'preserved'?

You're stating the MSR is per-*CORE*, and then continue to talk about
per-task state.

We've had a bunch of MSRs have exactly that problem recently, and it's
not fun. We're not going to do that again.