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Date:   Tue, 14 Jul 2020 21:49:21 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Ira Weiny <ira.weiny@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>, 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>,
        Fenghua Yu <fenghua.yu@intel.com>, 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 12/15] kmap: Add stray write protection for device
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On Tue, Jul 14, 2020 at 12:42:11PM -0700, Dave Hansen wrote:
> On 7/14/20 12:29 PM, Peter Zijlstra wrote:
> > On Tue, Jul 14, 2020 at 12:06:16PM -0700, Ira Weiny wrote:
> >> On Tue, Jul 14, 2020 at 10:44:51AM +0200, Peter Zijlstra wrote:
> >>> So, if I followed along correctly, you're proposing to do a WRMSR per
> >>> k{,un}map{_atomic}(), sounds like excellent performance all-round :-(
> >> Only to pages which have this additional protection, ie not DRAM.
> >>
> >> User mappings of this memory is not affected (would be covered by User PKeys if
> >> desired).  User mappings to persistent memory are the primary use case and the
> >> performant path.
> > Because performance to non-volatile memory doesn't matter? I think Dave
> > has a better answer here ...
> 
> So, these WRMSRs are less evil than normal.  They're architecturally
> non-serializing instructions,

Excellent, that should make these a fair bit faster than regular MSRs.

> But, either way, this *will* make accessing PMEM more expensive from the
> kernel.  No escaping that. 

There's no free lunch, it's just that regular MSRs are fairly horrible.