Re: [RFC PATCH 00/29] arm64: Scalable Vector Extension core support

[Florian Weimer <fweimer@redhat.com>]

On 11/25/2016 08:38 PM, Dave Martin wrote:
> The Scalable Vector Extension (SVE) [1] is an extension to AArch64 which
> adds extra SIMD functionality and supports much larger vectors.
>
> This series implements core Linux support for SVE.
>
> Recipents not copied on the whole series can find the rest of the
> patches in the linux-arm-kernel archives [2].
>
>
> The first 5 patches "arm64: signal: ..." factor out the allocation and
> placement of state information in the signal frame.  The first three
> are prerequisites for the SVE support patches.
>
> Patches 04-05 implement expansion of the signal frame, and may remain
> controversial due to ABI break issues:
>
>  * Discussion is needed on how userspace should detect/negotiate signal
>    frame size in order for this expansion mechanism to be workable.

I'm leaning towards a simple increase in the glibc headers (despite the 
ABI risk), plus a personality flag to disable really wide vector 
registers in case this causes problems with old binaries.

A more elaborate mechanism will likely introduce more bugs than it makes 
existing applications working, due to its complexity.

> The remaining patches implement initial SVE support for Linux, with the
> following limitations:
>
>  * No KVM/virtualisation support for guests.
>
>  * No independent SVE vector length configuration per thread.  This is
>    planned, but will follow as a separate add-on series.

Per-thread register widths will likely make coroutine switching 
(setcontext) and C++ resumable functions/executors quite challenging.

Can you detail your plans in this area?

Thanks,
Florian