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Re: [PATCH] ARM: Add support for AT_HWCAP2 in _dl_procinfo
- From: Richard Earnshaw <rearnsha at arm dot com>
- To: Will Newton <will dot newton at linaro dot org>
- Cc: "libc-alpha at sourceware dot org" <libc-alpha at sourceware dot org>
- Date: Thu, 26 Jun 2014 11:01:34 +0100
- Subject: Re: [PATCH] ARM: Add support for AT_HWCAP2 in _dl_procinfo
- Authentication-results: sourceware.org; auth=none
- References: <1403701970-21947-1-git-send-email-will dot newton at linaro dot org> <53ABE45B dot 1040303 at arm dot com> <CANu=DmiLJJkasLnJ7LJtHaL7Aco=BsoD98h7hMcqKDpL3QHoGQ at mail dot gmail dot com>
On 26/06/14 10:36, Will Newton wrote:
> On 26 June 2014 10:14, Richard Earnshaw <firstname.lastname@example.org> wrote:
>> On 25/06/14 14:12, Will Newton wrote:
>>> Add support for the new HWCAP2 values for ARMv8 added in the
>>> 3.15 kernel. Tested using QEMU which supports these extensions.
>>> 2014-06-25 Will Newton <email@example.com>
>>> * sysdeps/unix/sysv/linux/arm/dl-procinfo.c
>>> (_dl_arm_cap_flags): Add HWCAP2 values.
>>> * sysdeps/unix/sysv/linux/arm/dl-procinfo.h
>>> (_DL_HWCAP_COUNT): Increase to 37.
>>> (_DL_HWCAP_LAST): New define.
>>> (_DL_HWCAP2_LAST): New define.
>>> (_dl_procinfo): Add support for printing
>>> AT_HWCAP2 entries.
>>> (_dl_string_hwcap): Use _dl_hwcap_string.
>> I don't have a specific comment about this patch.
>> I do have a general comment that I think the HWCAPs exported by the
>> kernel for 32-bit ARM are a joke. The principle problem is that there
>> is precisely zero way to determine the base architecture. You cannot
>> even tell whether you are running on ARMv6 or ARMv7, let alone whether
>> you have key features such as Thumb2.
> I agree, and the situation on AArch64 looks no better. It's pretty
> much impossible to determine the micro-architecture from userland too
> - which may not be that much of an issue for ARM, but AArch64 likely
> much more so
There has to be a better way of addressing that issue than reading the
microarchitecture name and then switching on that. The list is
potentially unbounded: what do you do when you encounter a new
>> I've heard it suggested that you can part the architecture string (eg
>> armv7l), but 1) the format of this string is not precisely defined in a
>> way that allows you to predict what future cores will generate and 2)
>> parsing strings in ifunc code when function calls can't be made is
>> likely to be hairy at best.
> I think the platform is probably the best way to pass that info. The
> kernel currently sets it to:
> snprintf(elf_platform, ELF_PLATFORM_SIZE, "%s%c",
> list->elf_name, ENDIANNESS);
> Where elf_name is one of:
> That doesn't look too intractable, and we can work with the kernel
> guys to make sure nothing too surprising is added there.
Until you realize that these do not have a total ordering; that is,
while you can write
v4 < v5 < v5t < v6 < v7
You cannot insert v7m in that list at any point, since it is both more
and less than v6. In fact, it's both more and less than the baseline
v7, since it also has a divide instruction.
> The string
> parsing of architecture revision is pretty trivial in those cases.
> Perhaps this is something we can discuss at the GNU Tools Cauldron
> next month.
> On AArch64 the platform string is hardcoded to "aarch64" or "aarch64_be". :-/
Yeah, but then, I don't think reading this string is a useful way of
solving this problem.