Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 14 Jul 2020 21:05:39 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ira Weiny <ira.weiny@intel.com>
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
 switch
Message-ID: <20200714190539.GG5523@worktop.programming.kicks-ass.net>
References: <20200714070220.3500839-1-ira.weiny@intel.com>
 <20200714070220.3500839-5-ira.weiny@intel.com>
 <20200714082701.GO10769@hirez.programming.kicks-ass.net>
 <20200714185322.GB3008823@iweiny-DESK2.sc.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200714185322.GB3008823@iweiny-DESK2.sc.intel.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.