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

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Jun 22, 2023 at 06:11:20PM +0100, Mark Brown wrote:
> 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.

OK, so if the signal handler invokes a function that touches the ZA
state, it may use TPIDR2 for lazy saving in any callee. In this case we
need to restore the original TPIDR2 of the interrupted context on
sigreturn.

So I convinced myself this is the only option that makes sense ;). I'll
queue the patches.

-- 
Catalin