linux-kernel - Re: [PATCH v4 6/6] arch, x86: pmem api for ensuring durability of persistent memory updates

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Date:	Wed, 17 Jun 2015 08:08:49 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Christoph Hellwig <hch@....de>, Arnd Bergmann <arnd@...db.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Juergen Gross <jgross@...e.com>, X86 ML <x86@...nel.org>,
	"Kani, Toshimitsu" <toshi.kani@...com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Luis Rodriguez <mcgrof@...e.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stefan Bader <stefan.bader@...onical.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Michael Ellerman <mpe@...erman.id.au>,
	Tejun Heo <tj@...nel.org>, Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH v4 6/6] arch, x86: pmem api for ensuring durability of
 persistent memory updates

On Wed, Jun 17, 2015 at 7:54 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Wed, Jun 17, 2015 at 4:31 AM, Christoph Hellwig <hch@....de> wrote:
>> This mess with arch_ methods and an ops vecor is almost unreadable.
>>
>> What's the problem with having something like:
>>
>> pmem_foo()
>> {
>>         if (arch_has_pmem)              // or sync_pmem
>>                 arch_pmem_foo();
>>         generic_pmem_foo();
>> }
>>
>> This adds a branch at runtime, but that shoudn't really be any slower
>> than an indirect call on architectures that matter.
>
> No doubt it's premature optimization, but it bothered me that we'll
> end up calling cpuid perhaps multiple times every i/o.  If it's just a
> readability concern I could wrap it in helpers.  Getting it upstream
> is my primary concern at this point so I have no strong attachment to
> the indirect calls if that's all that is preventing an ack.

A cpuid per i/o would be a killer, but the cpufeature code is way
smarter than that.

You want static_cpu_has, though -- it's even faster, since it gets
patched at boot time.

--Andy
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