Re: [PATCH] KVM: PPC: Book3S PR: Do not always inject facility unavailable exceptions

[Paul Mackerras <paulus@ozlabs.org>]

On Mon, Apr 03, 2017 at 01:28:34PM +0200, Thomas Huth wrote:
> KVM should not inject a facility unavailable exception into the guest
> when it tries to execute a mtspr/mfspr instruction for an SPR that
> is unavailable, and the vCPU is *not* running in PRoblem state.
> 
> It's right that we inject an exception when the vCPU is in PR mode, since
> chapter "6.2.10 Facility Status and Control Register" of the PowerISA
> v2.07 says that "When the FSCR makes a facility unavailable, attempted
> usage of the facility in *problem state* is treated as follows: [...]
> Access of an SPR using mfspr/mtspr causes a Facility Unavailable
> interrupt". But if the guest vCPU is not in PR mode, we should follow
> the behavior that is described in chapter "4.4.4 Move To/From System
> Register Instructions" instead and treat the instruction as a NOP.

This doesn't seem quite right.  My reading of the ISA is that the FSCR
bit for a facility being 0 doesn't prevent privileged code from
accessing the facility, so we shouldn't be treating mfspr/mtspr as
NOP.  Instead we should be set the facility's bit in the shadow
FSCR and re-execute the instruction (remembering of course to clear
the FSCR bit when we go back to emulated problem state).

For TM it's a bit different as the MSR[TM] bit does prevent privileged
code from accessing TM registers and instructions, so for TM we should
be delivering a facility unavailable interrupt even when the guest is
in emulated privileged state.

So I don't see any case where mfspr/mtspr should be treated as a NOP
in response to a facility unavailable interrupt.

Paul.