linux-kernel - Re: [RFC 12/12] iscsi-target: Add Makefile/Kconfig and update TCM top level

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Message-Id: <1299185905.5401.405.camel@haakon2.linux-iscsi.org>
Date:	Thu, 03 Mar 2011 12:58:25 -0800
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Randy Dunlap <rdunlap@...otime.net>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org,
	James Bottomley <James.Bottomley@...senPartnership.com>
Subject: Re: [RFC 12/12] iscsi-target: Add Makefile/Kconfig and update TCM
	top level

On Thu, 2011-03-03 at 09:19 -0500, Christoph Hellwig wrote:
> On Wed, Mar 02, 2011 at 01:32:11PM -0800, Nicholas A. Bellinger wrote:
> > The kernel code itself that is specific to using the SSE v4.2
> > instruction for CRC32C offload are using #ifdef CONFIG_X86 stubs in
> > iscsi_target_login.c:iscsi_login_setup_crypto(), and !CONFIG_X86 will
> > default to using the unoptimized 1x8 slicing soft CRC32C code.  This
> > particular piece of logic has been tested on powerpc and arm and is
> > funcitoning as expected from the kernel level using the arch independent
> > soft code.
> 
> I don't think you need that code at all.  The crypto code is structured
> to prefer the optimized implementation if it is present.  Just stripping
> the x86-specific code out and always requesting the plain crc32c
> algorithm should give you the optimized one if it is present on your
> system.
> 
> Please give it a try.
> 

This is what I originally thought as well, but this ended up not being
the case when the logic was originally coded up.   I just tried again
with .38-rc7 on a 5500 series machine and simply stubbing out the
CONFIG_X86 part from iscsi_login_setup_crypto() and calling:

   crypto_alloc_hash("crc32c", 0, CRYPTO_ALG_ASYNC)

does not automatically load and use crc32c_intel.ko when only requesting
plain crc32c.

The reason for the extra crypto_alloc_hash("crc32c-intel", ...) call in
iscsi_login_setup_crypto() is to load crc32c_intel.ko on-demand for
cpu_has_xmm4_2 capable machines.

I should mention this is with the following .config:

CONFIG_CRYPTO_CRC32C=y
CONFIG_CRYPTO_CRC32C_INTEL=m

This would seem to indicate that CRC32C_INTEL needs to be compiled in
(or at least manually loaded) for libcypto to use the optimized
instructions when the plain crc32c is called, correct..?

--nab

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