[PATCH] arm64: Add support for Half precision floating point

[Adhemerval Zanella]

On 02-02-2016 15:31, Szabolcs Nagy wrote:
> On 28/01/16 16:51, Adhemerval Zanella wrote:
>> On 28-01-2016 14:07, Will Deacon wrote:
>>> On Tue, Jan 26, 2016 at 10:25:38PM +0530, Siddhesh Poyarekar wrote:
>>>> Adding Adhemerval to cc since he had volunteered to follow up on this,
>>>> mainly because he had a couple of additional ideas on the kernel
>>>> front.
>>>>
>>>> On Tue, Jan 26, 2016 at 04:21:43PM +0000, Suzuki K. Poulose wrote:
>>>>> On 26/01/16 16:02, Will Deacon wrote:
>>>>>> Hi Suzuki,
>>>>>>
>>>>>> On Tue, Jan 26, 2016 at 03:52:46PM +0000, Suzuki K Poulose wrote:
>>>>>>> ARMv8.2 extensions [1] include an optional feature, which supports
>>>>>>> half precision(16bit) floating point/asimd data processing
>>>>>>> instructions. This patch adds support for detecting and exposing
>>>>>>> the same to the userspace via HWCAPs
>>>>>
>>>>>
>>>>>>> +#define HWCAP_FPHP		(1 << 9)
>>>>>>> +#define HWCAP_ASIMDHP		(1 << 10)
>>>>>>
>>>>>> Where did we get to with the mrs trapping you proposed here?
>>>>>>
>>>>>>   http://lists.infradead.org/pipermail/linux-arm-kernel/2015-October/374609.html
>>>>>
>>>>> We are yet to get some feedback from glibc/gcc folks. Siddhesh was looking
>>>>> to make use of it [2]. But haven't heard anything back. Ramana mentioned
>>>>> (in private) that they had some plans to take a look at it.
>>>>
>>>> I believe one of Adhemerval's ideas was similar to what I had
>>>> mentioned back then, which was to provide all of the CPU information
>>>> in a single file instead of having to traverse a directory structure.
>>>
>>> My understanding was that libc needed this information extremely early
>>> on (i.e. before it could even issue system calls), and therefore such
>>> an approach would be in addition to the proposal here. Am I mistaken?
>>
>> If the idea is to use these instruction for function implementation selection
>> (iFUNC) the idea is to have on PLT resolution either by accessing it directly
>> or using a caching mechanism. x86_64 does something similar with cacheline
>> information: it issues a single cpuid and create processor information table
>> based on its information (it is also what the __builtin_supports() also
>> does).
>>
> 
> __builtin_supports is not a single cpuid on x86, it is
> a cpuid per dso with one cache per dso.
> 
> (gcc-5 used a single cache in libgcc_s.so.1 and that
> turned out to be broken because ifunc in other dsos
> could not reliably access it.)

It is with static libgcc (default), but if you use -shared-gcc only one
__cpu_model (used by __builtin_cpu_supports) will be linked.  But since
static libgcc is default it will be indeed one per DSO.

> 
>>>> The other idea was to add a vDSO function that returns this data so as
>>>> to avoid (or at least reduce) the context switch latency.
>>>
>>> I'm not at all keen on adding a data ABI to the vDSO. I think people tried
>>> similar things in the past (something on PPC?) and have horror stories
>>> from that.
>>
>> In fact ppc still exports it in vDSO (include/asm/vdso_datapage.h), with
>> information like the LPAR cfg, platform, processor, {d,i}cache, etc.
>> I recall that I have see some code back at IBM that tried to use these
>> fields directly, but indeed it is not recommended.
>>
>> What I have in mind is something what ppc does with __kernel_get_syscall_map.
>> It is vDSO function that returns a vDSO internal data related to which
>> syscalls are implemented in the running kernel (through a bitmap field).
>>
> 
> fs access or vdso does not work for ifunc based dispatch
> (assuming the current ifunc implementation in glibc).
> 
> (for vdso you need the AT_SYSINFO_EHDR auxval somehow and
> then implement elf symbol lookup in the ifunc resolver
> without calling any libc function. passing auxvals to the
> ifunc resolver can be done by changing the ifunc abi, but
> doing symbol lookups there is unrealistic.)
> 
> in the libc (e.g. for memcpy) ifunc is a bit easier to use,
> but in user code (function-multi-versioning) ifunc is very
> limited.
> 
> i wrote about the ifunc limitations here:
> https://sourceware.org/ml/libc-alpha/2015-11/msg00108.html
> see point (4) and (5).
> 

I recall this thread and indeed iFUNC have a set of limitations.  Although for
use within libc itself it might be safe with the constraints you have described.

Now for vDSO usage I think it might be safe to use within GLIBC
with correct vDSO pointers initialization order. At least it is done
on GLIBC for gettimeofday for x86_64 and powerpc (the iFUNC returns
the vDSO function pointer).