On Wed, Sep 20, 2017 at 06:08:21PM +0000, Alan Hayward wrote:

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On 20 Sep 2017, at 12:09, Dave Martin [off-list ref] wrote:

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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