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.
