linux-kernel - Re: [PATCH 00/13] deprecate pcommit

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Message-ID: <CALCETrVfxGYjL7mD95ku5E+nPeSKXdGPj+kJjjp=BHr5_1T3dg@mail.gmail.com>
Date:	Sun, 5 Jun 2016 10:41:26 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	linux-nvdimm <linux-nvdimm@...ts.01.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Dave Chinner <david@...morbit.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	Christoph Hellwig <hch@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>, "H. Peter Anvin" <hpa@...or.com>,
	X86 ML <x86@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 00/13] deprecate pcommit

On Jun 4, 2016 1:53 PM, "Dan Williams" <dan.j.williams@...el.com> wrote:
>
> Platforms supporting NVDIMMs are now required to provide persistence
> guarantees once pmem stores are accepted by the memory subsystem.

Can you point us to a precise definition of what exactly constitutes
stores being "accepted by the memory subsystem"?  Back when pcommit
was a thing (hah!), it was precisely documented in the SDM.

> is usually achieved by a platform-level feature known as ADR
> (Asynchronous DRAM Refresh) that flushes any memory subsystem write
> pending queues on power loss/shutdown.

Blech.  Does this mean that we need some NMI handler or similar that
does wbinvd?  Will CPU caches automatically write themselves back on
power loss?  Will we lose data if something goes wrong with an SMI
handler?

>
> The 'pcommit' instruction (which has not shipped on any product) is no
> longer needed and is deprecated.

Does this mean it will never ship?

--Andy