Re: [PATCH] [BZ #21391] x86: Set dl_platform and dl_hwcap from CPU features

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Thu, Apr 20, 2017 at 7:42 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Thu, Apr 20, 2017 at 1:51 AM, Florian Weimer <fweimer@redhat.com> wrote:
>> On 04/19/2017 09:31 PM, H.J. Lu wrote:
>>>
>>> On Wed, Apr 19, 2017 at 12:02 PM, Florian Weimer <fweimer@redhat.com>
>>> wrote:
>>>>
>>>> On 04/19/2017 08:35 PM, H.J. Lu wrote:
>>>>>
>>>>>
>>>>> dl_platform and dl_hwcap are set from AT_PLATFORM and AT_HWCAP very
>>>>> early during startup.  They are used by dynamic linker to determine
>>>>> platform and build an array of hardware capability names, which are
>>>>> added to search path when loading shared object.  dl_platform and
>>>>> dl_hwcap are unused on x86-64.  On i386, i386, i486, i586 and i686
>>>>> platforms were supported and only SSE2 capability was used.
>>>>
>>>>
>>>>
>>>> I don't know where you want to take this, so it's hard to tell if this is
>>>> going to cause problems eventually.  GLRO is default-initialized in a
>>>> nested
>>>> libc (after dlmopen or static dlopen).
>>>>
>>>
>>> dl_platform and dl_hwcap are used to search for additional directories
>>> when
>>> loading a shared object:
>>>
>>>       15453: find library=libx.so [0]; searching
>>>       15453: search path=./tls/x86_64:./tls:./x86_64:. (RPATH from file
>>> ./m)
>>>       15453:  trying file=./tls/x86_64/libx.so
>>>       15453:  trying file=./tls/libx.so
>>>       15453:  trying file=./x86_64/libx.so
>>>       15453:  trying file=./libx.so
>>>       15453:
>>>
>>> My change updates them inside dynamic linker before they are used
>>> by dynamic linker.  On Haswell class machines, I got
>>>
>>>       19268: find library=libx.so [0]; searching
>>>       19268: search path=./tls/haswell:./tls:./haswell:. (RPATH from file
>>> ./m)
>>>       19268:  trying file=./tls/haswell/libx.so
>>>       19268:  trying file=./tls/libx.so
>>>       19268:  trying file=./haswell/libx.so
>>>
>>> When loading libx.so, it prefers the one in the "haswell" subdirectory.
>>> One can place shared libraries optimized for Haswell class processors
>>> under the "haswell" subdirectory.  They will be used on Haswell class
>>> processors.  It has no impact on static dlopen nor nested libc.
>>
>>
>> Okay, but only because the external dynamic linker is used, which has a
>> fully set up GLRO object.  The default-initialized GLRO object in the inner
>> libc is not used in this case.
>
> That is true.  My patch only applies to the external dynamic linker.
>

Any other comments?


-- 
H.J.