[PATCH v2 1/2] arm64/signal: Restore TPIDR2 register rather than memory state

[Mark Brown]

On Thu, Jun 22, 2023 at 05:42:54PM +0100, Catalin Marinas wrote:
> On Thu, Jun 22, 2023 at 02:39:45PM +0100, Mark Brown wrote:

> > -		current->thread.tpidr2_el0 = tpidr2_el0;
> > +		write_sysreg_s(tpidr2_el0, SYS_TPIDR2_EL0);

> I guess the other way around may also be true - the libc sets tpidr2_el0
> to something else and doesn't want the kernel to restore its original
> value from sigcontext.

> For tpidr_el0 we don't bother with sigcontext, not sure what the use for
> tpidr2_el0 in signals is. If we assume the context saved is only
> informative (like esr), we can simply ignore restoring it from the
> signal stack.

TPIDR2 is intended to go along with the thread stack, it's intended to
be used to allow lazy save of the (rather large) ZA register state when
a called function needs it rather than forcing it to be caller saved.
TPIDR2 is used to point to memory allocated for managing this process,
something that provides a new value should be making a deliberate
decision to do so and editing the stack frame.

> I guess we need to ask Szabolcs what his preference is. The current code
> is wrong either way since current->thread.tpidr2_el0 would be overridden
> at thread switch.

Right.
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