Re: [PATCH v2 2/4] arm64/ptrace: Document extension of NT_ARM_TLS to cover TPIDR2_EL0

[Mark Brown <broonie@kernel.org>]

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On Thu, Aug 18, 2022 at 10:17:11AM +0100, Luis Machado wrote:
> On 8/15/22 14:30, Mark Brown wrote:

> > +* The NT_ARM_TLS note will be extended to two registers, the second register
> > +  will contain TPIDR2_EL0 on systems that support SME and will be read as
> > +  zero with writes ignored otherwise.

> I wonder if we should document a bit more about the use of TPIDR2, its states, values and
> block format when TPIDR2 points to valid ZA state.

> Would that make sense?

That seems somewhat out of scope for the kernel, it's doesn't have any
idea about the use of TPIDR2 and I'm not sure we want to give anyone the
impression that it might try to do anything clever with it.  From the
perspective of the kernel ABI this is simply another TPIDR that needs to
be context switched, I have heard some suggestions that the kernel
should try to do a spill to the TPIDR2 block allocated in user memory
directly but that feels like it's getting a bit more hairy than we want
and problematic for anyone doing off piste with regard to ABI.

Also note that the spec for TPIDR2 didn't get merged into the PCS yet so
it's not quite finalised.

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