Re: [PATCH 1/2] aarch64: Remove HWCAP_CPUID from HWCAP_IMPORTANT

[Siddhesh Poyarekar <siddhesh at gotplt dot org>]

On 06/28/2018 11:52 PM, Szabolcs Nagy wrote:
> This partially reverts
>
> commit f82e9672ad89ea1ef40bbe1af71478e255e87c5e
> Author:     Siddhesh Poyarekar <siddhesh@sourceware.org>
>
>      aarch64: Allow overriding HWCAP_CPUID feature check using HWCAP_MASK
>
> The idea was to make it possible to disable cpuid based ifunc resolution
> in glibc by changing the hwcap mask which the user could already control.
>
> However the hwcap mask has an orthogonal role: it specifies additional
> library search paths for the dynamic linker.  So "cpuid" got added to

I don't think that is correct[1]; I understood hwcap_mask to be a 
general tool that allows you to mask out hardware capabilities as needed 
and the library search paths feature happens to be a user of this. 
Another user is elf_machine_matches_host in sparc code for example, 
which uses to match the binary with supported hardware capabilities.

> Meanwhile there is a tunable to set the cpu explicitly so it is possible
> to disable the cpuid based dispatch without using a hwcap mask:

However, I'm not too attached to the hwcap_mask way of disabling CPUID, 
so removing the aarch64 check is OK with me.  It was a good quick option 
to implement back when the tune.cpu tunable was not in (and they 
eventually went in together IIRC, so it was a moot point) but I agree 
that it is redundant now.

That said, it would be nice to hear from others (especially distro 
folks) since it may have been advertised as a way to disable the ifunc 
selection.

Siddhesh

[1] I don't have the historical reference to assert that it is 
definitely wrong, so if you have one I'll happily correct myself.