Subject: Re: [PATCH v3 02/52] powerpc/64s: guard optional TIDR SPR with CPU ftr test

From: Michael Ellerman <[email-masked]>
Date: Tue, 12 Oct 2021 13:08:57 +1100

Fabiano Rosas <[email-masked]> writes:
> Nicholas Piggin <[email-masked]> writes:
>
>> The TIDR SPR only exists on POWER9. Avoid accessing it when the
>> feature bit for it is not set.
>
> Not related to this patch, but how does this work with compat mode? A P9
> compat mode guest would get an invalid instruction when trying to access
> this SPR?

Good question.

I assume you're talking about P9 compat mode on P10.

In general compat mode only applies to userspace, because it's
implemented by setting the PCR which only (mostly?) applies to PR=1.

I don't think there's any special casing in the ISA for the TIDR, so I
think it just falls into the unimplemented SPR case for mt/fspr.

That's documented in Book III section 5.4.4, in particular on page 1171
it says:

  Execution of this instruction specifying an SPR number
  that is undefined for the implementation causes one of
  the following.
  • if spr[0]=0:
    - if MSR[PR]=1: Hypervisor Emulation Assistance interrupt
    - if MSR[PR]=0: Hypervisor Emulation Assistance interrupt for SPR
      0,4,5, and 6, and no operation (i.e., the instruction is treated
      as a no-op) when LPCR[EVIRT]=0 and Hypervisor Emulation Assistance
      interrupt when LPCR[EVIRT]=1 for all other SPRs

Linux doesn't set EVIRT, and I assume neither does phyp, so it behaves
like a nop.

We actually use that behaviour in xmon to detect that an SPR is not
implemented, by noticing that the mfspr has no effect on the target
register, see dump_one_spr().

We should really write some docs on compat mode in the linuxppc wiki
and/or Documentation ;)

cheers