[PATCH] AArch64: Check kernel version for SVE ifuncs

[Florian Weimer]

* Wilco Dijkstra:

> Older Linux kernels may disable SVE after certain syscalls.  Calling the
> SVE-optimized memcpy afterwards will then cause a trap to reenable SVE.
> As a result, applications with a high use of syscalls may run slower with
> the SVE memcpy.  Avoid this by checking the kernel version and enable the
> SVE-optimized memcpy/memmove ifuncs only on Linux kernel 6.2 or newer.

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.

> +/* Parse kernel version without calling any library functions.
> +   Allow 2 digits for kernel version and 3 digits for major version,
> +   separated by '.': "kk.mmm.".
> +   Return kernel version * 1000 + major version, or -1 on failure.  */

I think you should parse the versions you exclude because you know their
syntax.  We tried to predict the layout of future kernel version strings
before and failed.

Thanks,
Florian