[PATCH] crypto: arm64/crc32 - detect crc32 support in assembler

On 1 February 2017 at 13:58, Alexander Graf [off-list ref] wrote:

On 02/01/2017 10:43 AM, Ard Biesheuvel wrote:

quoted

On 1 February 2017 at 09:07, Ard Biesheuvel [off-list ref]
wrote:

quoted

On 27 January 2017 at 10:52, Will Deacon [off-list ref] wrote:

quoted

On Fri, Jan 27, 2017 at 10:43:16AM +0000, Ard Biesheuvel wrote:

quoted

On 27 January 2017 at 10:40, Matthias Brugger [off-list ref]
wrote:

quoted

Older compilers may not be able to detect the crc32 extended cpu type.

What do you mean 'detect'? Could you describe the failure in more
detail
please?

quoted

Anyway only inline assembler code is used, which gets passed to the
assembler. This patch moves the crc detection to the assembler.

Suggested-by: Alexander Graf <redacted>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>
---
  arch/arm64/crypto/Makefile      | 2 --
  arch/arm64/crypto/crc32-arm64.c | 3 +++
  2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/crypto/Makefile b/arch/arm64/crypto/Makefile
index aa8888d7b744..0d779dac75cd 100644
--- a/arch/arm64/crypto/Makefile
+++ b/arch/arm64/crypto/Makefile

@@ -48,8 +48,6 @@ CFLAGS_aes-glue-ce.o  := -DUSE_V8_CRYPTO_EXTENSIONS

  obj-$(CONFIG_CRYPTO_CRC32_ARM64) += crc32-arm64.o

-CFLAGS_crc32-arm64.o   := -mcpu=generic+crc
-
  $(obj)/aes-glue-%.o: $(src)/aes-glue.c FORCE
         $(call if_changed_rule,cc_o_c)

diff --git a/arch/arm64/crypto/crc32-arm64.c

b/arch/arm64/crypto/crc32-arm64.c
index 6a37c3c6b11d..10f5dd075323 100644

--- a/arch/arm64/crypto/crc32-arm64.c
+++ b/arch/arm64/crypto/crc32-arm64.c

@@ -29,6 +29,9 @@ MODULE_AUTHOR("Yazen Ghannam

[off-list ref]");
  MODULE_DESCRIPTION("CRC32 and CRC32C using optional ARMv8
instructions");
  MODULE_LICENSE("GPL v2");

+/* Request crc extension capabilities from the assembler */
+asm(".arch_extension crc");
+

Will should confirm, but I think this is a recent feature in GAS for
AArch64, so this may break older toolchains as well.

Yes, the .arch_extension directive isn't universally supported by
AArch64
gas so we can't rely on it unconditionally. The best bet is to check for
the support and, if it's not present, then disable whatever feature
relies
on it. See the lseinstr variable in Makefile.

Actually, this driver has become somewhat redundant now that we have
an alternative that combines an implementation based on 64x64
polynomial multiplication with an implementation based on the CRC32
instructions.

I will propose a patch that makes the latter usable when only the
CRC32 instructions are supported.

... although you still haven't explained what the actual problem is
that you are trying to solve.

The problem is that in Leap 42.2 (as well as SLES12 SP2) we have a 4.8 based
system compiler, but recent binutils. That means that while our assembler is
happy to work with crc instructions, passing the -mcpu parameter to gcc
fails because gcc isn't aware of the flavor yet.

That in turn means that we want to tell the assembler about feature
requirements rather than the compiler. Fortunately the ".arch_extension"
primitive allows you to do so.

As far as checking for availability of it goes, I agree that it'd be nice to
check if ".arch_extension" is supported. But so would be to check if
-mcpu=generic+crc is supported. IMHO this patch doesn't make the current
non-checking situation any worse. But of course checking is always nicer
than not checking :)

Well, I am pretty sure binutils v2.25 and older are in wider use than
GCC v4.8 and older, which means the runtime test would disable the CRC
module altogether for a lot of users.

However, as I mentioned, we can also remove this driver once the PMULL
based one is updated.