[PATCH v6 3/3] arm64: pac: Optimize kernel entry/exit key installation code paths

[Will Deacon]

On Tue, Dec 29, 2020 at 10:59:15PM -0800, Peter Collingbourne wrote:
> The kernel does not use any keys besides IA so we don't need to
> install IB/DA/DB/GA on kernel exit if we arrange to install them
> on task switch instead, which we can expect to happen an order of
> magnitude less often.
> 
> Furthermore we can avoid installing the user IA in the case where the
> user task has IA disabled and just leave the kernel IA installed. This
> also lets us avoid needing to install IA on kernel entry.

I've got to be honest, this makes me nervous in case there is a way for
userspace to recover the kernel key even though EnIA is clear. Currently,
EnIA doesn't affect XPAC* and PACGA instructions, and the architecture
clearly expects us to be switching these things:

  | Note
  | Keys are not banked by Exception level. Arm expects software to switch the
  | keys between Exception levels, typically by swapping the values with zero
  | so that the current key values are not present in memo

But then:

> On an Apple M1 under a hypervisor, the overhead of kernel entry/exit
> has been measured to be reduced by 15.6ns in the case where IA is
> enabled, and 31.9ns in the case where IA is disabled.

That's a good improvement, so this feels like its worth doing. I suppose all we
can do is keep an eye on the architecture in case any future extensions mean
the approach taken here is dangerous.

Will