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Re: [RFC PATCH 00/29] arm64: Scalable Vector Extension core support
- From: Yao Qi <qiyaoltc at gmail dot com>
- To: Dave Martin <Dave dot Martin at arm dot com>
- Cc: Florian Weimer <fweimer at redhat dot com>, linux-arm-kernel at lists dot infradead dot org, libc-alpha at sourceware dot org, Ard Biesheuvel <ard dot biesheuvel at linaro dot org>, Marc Zyngier <Marc dot Zyngier at arm dot com>, gdb at sourceware dot org, Alan Hayward <alan dot hayward at arm dot com>, Torvald Riegel <triegel at redhat dot com>, Christoffer Dall <christoffer dot dall at linaro dot org>
- Date: Fri, 2 Dec 2016 21:56:46 +0000
- Subject: Re: [RFC PATCH 00/29] arm64: Scalable Vector Extension core support
- Authentication-results: sourceware.org; auth=none
- References: <7a35d1ae-73df-03a5-c9d6-1a52754acf25@redhat.com> <CAH=s-POegNL2HLV3EGPt=b-jrJ4OOwpBFTfysMaX0TNgSw=3Hg@mail.gmail.com> <20161130120654.GJ1574@e103592.cambridge.arm.com>
On 16-11-30 12:06:54, Dave Martin wrote:
> So, my key goal is to support _per-process_ vector length control.
>
> From the kernel perspective, it is easiest to achieve this by providing
> per-thread control since that is the unit that context switching acts
> on.
>
Hi, Dave,
Thanks for the explanation.
> How useful it really is to have threads with different VLs in the same
> process is an open question. It's theoretically useful for runtime
> environments, which may want to dispatch code optimised for different
> VLs -- changing the VL on-the-fly within a single thread is not
> something I want to encourage, due to overhead and ABI issues, but
> switching between threads of different VLs would be more manageable.
This is a weird programming model.
> However, I expect mixing different VLs within a single process to be
> very much a special case -- it's not something I'd expect to work with
> general-purpose code.
>
> Since the need for indepent VLs per thread is not proven, we could
>
> * forbid it -- i.e., only a thread-group leader with no children is
> permitted to change the VL, which is then inherited by any child threads
> that are subsequently created
>
> * permit it only if a special flag is specified when requesting the VL
> change
>
> * permit it and rely on userspace to be sensible -- easiest option for
> the kernel.
Both the first and the third one is reasonable to me, but the first one
fit well in existing GDB design. I don't know how useful it is to have
per-thread VL, there may be some workloads can be implemented that way.
GDB needs some changes to support "per-thread" target description.
--
Yao