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Re: [PATCH v2 14/28] arm64/sve: Backend logic for setting the vector length
- From: Dave Martin <Dave dot Martin at arm dot com>
- To: Alan Hayward <Alan dot Hayward at arm dot com>
- Cc: Dave Martin <dave dot martin at foss dot arm dot com>, "linux-arch at vger dot kernel dot org" <linux-arch at vger dot kernel dot org>, "libc-alpha at sourceware dot org" <libc-alpha at sourceware dot org>, "gdb at sourceware dot org" <gdb at sourceware dot org>, Ard Biesheuvel <ard dot biesheuvel at linaro dot org>, Szabolcs Nagy <Szabolcs dot Nagy at arm dot com>, Catalin Marinas <Catalin dot Marinas at arm dot com>, Yao Qi <Yao dot Qi at arm dot com>, Will Deacon <Will dot Deacon at arm dot com>, Richard Sandiford <Richard dot Sandiford at arm dot com>, nd <nd at arm dot com>, Alex Bennée <alex dot bennee at linaro dot org>, "kvmarm at lists dot cs dot columbia dot edu" <kvmarm at lists dot cs dot columbia dot edu>, "linux-arm-kernel at lists dot infradead dot org" <linux-arm-kernel at lists dot infradead dot org>
- Date: Thu, 21 Sep 2017 12:19:45 +0100
- Subject: Re: [PATCH v2 14/28] arm64/sve: Backend logic for setting the vector length
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- References: <1504198860-12951-1-git-send-email-Dave.Martin@arm.com> <1504198860-12951-15-git-send-email-Dave.Martin@arm.com> <FC5BA359-D37C-493C-87CD-146B83D3CCB5@arm.com> <20170920110902.GG24231@e103592.cambridge.arm.com> <E82895B2-3F95-4A54-8703-D654221F8CBA@arm.com>
On Wed, Sep 20, 2017 at 06:08:21PM +0000, Alan Hayward wrote:
>
> > On 20 Sep 2017, at 12:09, Dave Martin <dave.martin@foss.arm.com> wrote:
[...]
> >> Given, sve_set_vector_length is called when setting the vector length in
> >> PTRACE_SETREGSET, it looks to me like if you set VL to a value that’s not
> >> supported by the hardware, then it’s going to round down to the previous value.
> >> Is that correct? I’m not sure if that’s explained in the docs?
> >
> > Does this cover it?
> >
> > "On success, the calling thread's vector length is changed to the
> > largest value supported by the system that is less than or equal to vl."
> >
> > (For ptrace, I just cross-reference the PR_SVE_SET_VL behaviour, above.)
>
> For ptrace is it worth mentioning user should do a GET after a SET to confirm
> what VL value was actually set?
This seems worth a clarification -- I'd thought this was already
mentioned, but it isn't.
How about:
The caller must make a further GETREGSET call if it needs to know what VL is
actually set by SETREGSET, unless is it known in advance that the requested
VL is supported.
[...]
Cheers
---Dave