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Re: [PATCH] ARM: Add support for AT_HWCAP2 in _dl_procinfo
- From: Will Newton <will dot newton at linaro dot org>
- To: Richard Earnshaw <rearnsha at arm dot com>
- Cc: "libc-alpha at sourceware dot org" <libc-alpha at sourceware dot org>
- Date: Thu, 26 Jun 2014 14:07:21 +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> <53ABEF7E dot 3070909 at arm dot com>
On 26 June 2014 11:01, Richard Earnshaw <email@example.com> wrote:
> 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
In the context of an ifunc resolver, do what you do now which is use
the hwcap bits. The use case that I can imagine is where we have a
microarchitecture that suffers from a particularly poor behaviour with
e.g. the default memcpy then we can switch it to use a custom version.
At the moment the only way I can see to deal with that is read /proc
and stash that information inside ld.so somewhere at startup but that
is rather ugly...
>>> 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.
I don't see why we would care about ordering these values - isn't it
just a symbolic value? What use case do you have in mind?
>> 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.
Do you have an alternative proposal? ;-)
Toolchain Working Group, Linaro