From mboxrd@z Thu Jan 1 00:00:00 1970 From: qiyaoltc@gmail.com (Yao Qi) Date: Fri, 2 Dec 2016 21:56:46 +0000 Subject: [RFC PATCH 00/29] arm64: Scalable Vector Extension core support In-Reply-To: <20161130120654.GJ1574@e103592.cambridge.arm.com> References: <7a35d1ae-73df-03a5-c9d6-1a52754acf25@redhat.com> <20161130120654.GJ1574@e103592.cambridge.arm.com> Message-ID: <20161202215646.s2xymph6d6jlyrv3@localhost> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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