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From: Dave Hansen <dave.hansen@intel.com>
To: Peter Zijlstra <peterz@infradead.org>, 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>,
	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 pages
Date: Tue, 14 Jul 2020 12:42:11 -0700	[thread overview]
Message-ID: <50d472d8-e4d9-dd35-f31f-268aa69c76e2@intel.com> (raw)
In-Reply-To: <20200714192930.GH5523@worktop.programming.kicks-ass.net>

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, just like the others in the SDM WRMSR
documentation:

	Note that WRMSR to the IA32_TSC_DEADLINE MSR (MSR index 6E0H)
	and the X2APIC MSRs (MSR indices 802H to 83FH) are  not
	serializing.

This section of the SDM needs to be updated for the PKRS.  Also note
that the PKRS WRMSR is similar in its ordering properties to WRPKRU:

	WRPKRU will never execute speculatively. Memory accesses
	affected by PKRU register will not execute (even speculatively)
	until all prior executions of WRPKRU have completed execution
	and updated the PKRU register.

Which means we don't have to do silliness like LFENCE before WRMSR to
get ordering *back*.  This is another tidbit that needs to get added to
the SDM.  It should probably also get captured in the changelog.

But, either way, this *will* make accessing PMEM more expensive from the
kernel.  No escaping that.  But, we've also got customers saying they
won't deploy PMEM until we mitigate this stray write issue.  Those folks
are quite willing to pay the increased in-kernel cost for increased
protection from stray kernel writes.  Intel is also quite motivated
because we really like increasing the number of PMEM deployments. :)

Ira, can you make sure this all gets pulled into the changelogs somewhere?

  reply	other threads:[~2020-07-14 19:42 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-14  7:02 [RFC PATCH 00/15] PKS: Add Protection Keys Supervisor (PKS) support ira.weiny
2020-07-14  7:02 ` [RFC PATCH 01/15] x86/pkeys: Create pkeys_internal.h ira.weiny
2020-07-14  7:02 ` [RFC PATCH 02/15] x86/fpu: Refactor arch_set_user_pkey_access() for PKS support ira.weiny
2020-07-14  7:02 ` [RFC PATCH 03/15] x86/pks: Enable Protection Keys Supervisor (PKS) ira.weiny
2020-07-14  7:02 ` [RFC PATCH 04/15] x86/pks: Preserve the PKRS MSR on context switch ira.weiny
2020-07-14  8:27   ` Peter Zijlstra
2020-07-14 18:53     ` Ira Weiny
2020-07-14 18:56       ` Dave Hansen
2020-07-14 19:05       ` Peter Zijlstra
2020-07-14 19:09         ` Ira Weiny
2020-07-14  7:02 ` [RFC PATCH 05/15] x86/pks: Add PKS kernel API ira.weiny
2020-07-14  7:02 ` [RFC PATCH 06/15] x86/pks: Add a debugfs file for allocated PKS keys ira.weiny
2020-07-14  7:02 ` [RFC PATCH 07/15] Documentation/pkeys: Update documentation for kernel pkeys ira.weiny
2020-07-14  7:02 ` [RFC PATCH 08/15] x86/pks: Add PKS Test code ira.weiny
2020-07-14  7:02 ` [RFC PATCH 09/15] fs/dax: Remove unused size parameter ira.weiny
2020-07-14  7:02 ` [RFC PATCH 10/15] drivers/dax: Expand lock scope to cover the use of addresses ira.weiny
2020-07-14  7:02 ` [RFC PATCH 11/15] memremap: Add zone device access protection ira.weiny
2020-07-14  8:40   ` Peter Zijlstra
2020-07-14 19:10     ` Ira Weiny
2020-07-14 19:40       ` Peter Zijlstra
2020-07-14  7:02 ` [RFC PATCH 12/15] kmap: Add stray write protection for device pages ira.weiny
2020-07-14  8:44   ` Peter Zijlstra
2020-07-14 19:06     ` Ira Weiny
2020-07-14 19:29       ` Peter Zijlstra
2020-07-14 19:42         ` Dave Hansen [this message]
2020-07-14 19:49           ` Peter Zijlstra
2020-07-14 20:00           ` Ira Weiny
2020-07-14  7:02 ` [RFC PATCH 13/15] dax: Stray write protection for dax_direct_access() ira.weiny
2020-07-14  7:02 ` [RFC PATCH 14/15] nvdimm/pmem: Stray write protection for pmem->virt_addr ira.weiny
2020-07-14  7:02 ` [RFC PATCH 15/15] [dax|pmem]: Enable stray write protection ira.weiny

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