Re: [PATCH v2 14/28] arm64/sve: Backend logic for setting the vector length

Alan Hayward
2017-09-21T11:57:24Z

> On 21 Sep 2017, at 12:19, Dave Martin <Dave.Martin@arm.com> wrote:
>
> 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.
>

Looks good to me.

Alan.