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Date: Wed, 17 Jun 2015 07:54:14 -0700
Message-ID: <CAPcyv4hkS+3iTqJkcuES13vZKNYdWKufAqjD3+Pf4BaZ88nZEQ@mail.gmail.com>
Subject: Re: [PATCH v4 6/6] arch, x86: pmem api for ensuring durability of
 persistent memory updates
From: Dan Williams <dan.j.williams@intel.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Arnd Bergmann <arnd@arndb.de>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Juergen Gross <jgross@suse.com>, X86 ML <x86@kernel.org>,
        "Kani, Toshimitsu" <toshi.kani@hp.com>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Luis Rodriguez <mcgrof@suse.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Stefan Bader <stefan.bader@canonical.com>,
        Andy Lutomirski <luto@amacapital.net>, linux-mm@kvack.org,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Ralf Baechle <ralf@linux-mips.org>,
        Henrique de Moraes Holschuh <hmh@hmh.eng.br>, mpe@ellerman.id.au,
        Tejun Heo <tj@kernel.org>, Paul Mackerras <paulus@samba.org>
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On Wed, Jun 17, 2015 at 4:31 AM, Christoph Hellwig <hch@lst.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.
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