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Date:   Tue, 14 Jul 2020 12:09:45 -0700
From:   Ira Weiny <ira.weiny@intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Fenghua Yu <fenghua.yu@intel.com>, x86@kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-nvdimm@lists.01.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-kselftest@vger.kernel.org
Subject: Re: [RFC PATCH 04/15] x86/pks: Preserve the PKRS MSR on context
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On Tue, Jul 14, 2020 at 09:05:39PM +0200, Peter Zijlstra wrote:
> 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@intel.com wrote:
> > > > From: Ira Weiny <ira.weiny@intel.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.

Ah sorry, my mistake yes I meant 'per-logical-processor' like Dave said.  I'll
update the commit message.

Ira