Re: [PATCH] RFC: Provide a function to reset IFUNC PLTs

[Florian Weimer <fweimer@redhat.com>]

* Jan Kratochvil:

> On Tue, 07 Mar 2023 16:40:46 +0800, Florian Weimer wrote:
>> Use of <sys/platform/x86.h> is somewhat rare.  Not even GCC uses it
>> AFAIK.  It has its own cached CPU data used for target clones and
>> similar features.  Re-running those IFUNC resolvers will just give the
>> same results.
>
> I do not use <sys/platform/x86.h> anywhere, why do you mention it?
> I am doing there
> 	memset((volatile uint8_t *)cpu_features, 0, sizeof(*cpu_features));
> 	...
> 	_dl_x86_init_cpu_features();
> to prevent the cache of CPU data. And it does work.

This code resets the data queried by <sys/platform/x86.h>.  But GCC and
others do not use these interfaces, they use CPUID/XGETBV directly and
cache the result, so this reset does not alter the outcome of their
IFUNC resolvers.

>> Distributions have started to deploy alternate builds in
>> glibc-hwcaps/x86-64-v3 directories:
>> 
>>   openSUSE Tumbleweed gains optional x86-64-v3 optimization
>>   <https://news.opensuse.org/2023/03/02/tw-gains-optional-optimizations/>
>> 
>> This might be a common situation fairly soon.  This is not IFUNC-based
>> at all, so any IFUNC-based approach is not going to help with that.
>
> What about some "dlreopen" to replace the variant of glibc?

The trivial implementation is not going to work because most of the
function and data addresses will change as a result of the reload.  Most
checkpoints probably occur in a blocking system call, and that would
need some form of on-stack replacement to work properly.

>> Practically speaking, I think cluster heterogeneity needs to be hidden
>> at the cluster level.
>
> The question is how to implement it. I have found
> 	https://github.com/ddcc/libcpuidoverride
> but it is not compatible with latest glibc (maybe its loader code needs to
> implement ELF relocations, it would be definitely fixable).

It needs kernel support for sure.  I do not know to what extent CPUID
can be context-switched at the silicon level, maybe the masking is
system-wide (or per-guest in a virtualized configuration). 

> Another option is GLIBC_TUNABLES=glibc.cpu.hwcaps=... although then one has to
> translate cpuid feature bits into the glibc names like SSE4_2 etc.
> Also re-exec() after setenv("GLIBC_TUNABLES") is not too nice.

Again, that's not going to work in practice because libraries and
applications do not use <sys/platform/x86.h>.

Thanks,
Florian