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

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?


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