[RFC PATCH 0/5] arm64: Signal context expansion

[Florian Weimer]

On 09/12/2016 05:30 PM, Dave Martin wrote:
> On Mon, Sep 12, 2016 at 04:01:17PM +0100, Szabolcs Nagy wrote:
>> i assume the kernel can avoid saving SVE regs when
>> they are not used by the process.
>
> I can (and do), in my patches (not posted yet).
>
> The real issue here is that a recently updated shared library might be
> optimised to use SVE, where the program using it is an older, SVE-
> unaware binary.
>
> (think of an optimised math library using some new fancy SVE-based
> number crunching internally).

In my little world, binaries are expected to do run-time CPU feature 
discovery and fall back to the baseline ABI if the system does not 
support ABI extensions.  This means that a personality flag can disable 
the extension, the optimized math libraries run with the slower baseline 
code instead, and the rest of the application sees the expected kernel 
interface.

 From this perspective, it is quite important that it is possible to get 
legacy applications running on newer hardware, preferably with out 
wrapping them in a dedicated VM.

I understand that there are other environments were this is less of a 
concern, but I also believe that the age of an architecture factors less 
into this than one might want.

Thanks,
Florian