Re: [PATCH] AArch64: Check kernel version for SVE ifuncs

[Florian Weimer <fweimer@redhat.com>]

* Wilco Dijkstra:

> Hi Florian,
>
>> Given that the existing SVE users have not complained about this, is it
>> really a good idea to disable SVE string functions for them?  Not
>> everyone interleaves system calls and string functions in inner loops.
>
> People have found significant regressions when trying the SVE memcpy [1],
> the kernel commit mentions 70% overhead in microbenchmarks [2].
>
> Also distros appear to use a wide range of kernel versions. I've seen the
> same GLIBC version used with 5.11 kernel all the way up to 6.5 in the
> same version of a distro - and these are all standard installations...

I generally prefer we fix the component that has the bug.  With that
approach, you'd have to to use your distribution contacts to request a
backport.

The workaround introduces a performance hit for those users who today
benefit from the SVE string operations, but do not suffer from the
syscall interaction issue.

In the end, it is your port, but version-based workarounds are really
unusual for glibc.

Thanks,
Florian