On 05/16/2012 04:13 AM, Alexander Graf wrote: > On 05/16/2012 11:02 AM, Sethi Varun-B16395 wrote: >> >>> -----Original Message----- >>> From: kvm-ppc-ow...@vger.kernel.org [mailto:kvm-ppc- >>> ow...@vger.kernel.org] On Behalf Of Bhushan Bharat-R65777 >>> Sent: Wednesday, May 16, 2012 12:58 PM >>> To: Alexander Graf >>> Cc: Wood Scott-B07421; Yoder Stuart-B08248; kvm-ppc@vger.kernel.org >>> Subject: Not emulated registers on BOOKE_HV (GS-mode) >>> >>> Hi Alex, >>> >>> There is below comment in arch/powerpc/kvm/booke_emulate.c >>> >>> /* >>> * NOTE: some of these registers are not emulated on BOOKE_HV >>> (GS-mode). >>> * Their backing store is in real registers, and these functions >>> * will return the wrong result if called for them in another context >>> * (such as debugging). >>> */ >>> >>> "some of these registers are not emulated on BOOKE_HV (GS-mode)" >>> 1) Is not that mtspr()/mfspr() for "not emulated" registers should >>> follow EMULATE_FAIL path? So should be ifdef out for BOOKE_HV? Otherwise >>> the emulation code execute. >>> 2) Or These are not emulated because the GS mode have direct access to >>> these registers, Right? So no trap? >>> >> For BOOKE_HV mtspr/mfspr would get mapped to hardware maintained guest >> shadow copies. >> For example guest access to dear would get mapped to gdear and >> wouldn't trap >> >>> "and these functions will return the wrong result if called for them in >>> another context (such as debugging)." >>> 1) So do you mean that guest is not supposed to access these registers >>> in normal scenario but the debugger (some command on gdb in guest) can >>> access these register? then does it make sense to treat mtspr() as nop >>> and mfspr returns 0/undefined? >>> >> Actually, in a normal scenario (For BOOKE_HV) guest (MSR[GS] = 1) >> would be able to directly access these registers >> without hypervisor intervention. However I am not sure under what >> condition would the guest access generate a trap, all >> guest accesses would be with MSR[GS]=1. > > Yup. E500mc would just never reach that code - and even if it did, it > simply wouldn't do anything useful. > > What did you guys need this change for? I'm sure there's some incentive > behind it, right? The only one I can see right now would be reduced code > size.
This was something Stuart pulled out of my local working tree from a commit where the entire commit message was "crit ints and misc emulation stuff to be sorted through". :-P Looking through previous versions of the history, the ifdefs were how Varun originally added critical interrupt support. I probably changed it to the "NOTE:" version without ifdefs, and the "misc emulation stuff" patch was just waiting for me to integrate that change properly with things like crit ints that weren't part of the e500mc patchset I posted. That said, if this code ever does get used for debug or similar non-emulation access, it would be nicer to fail than to return garbage -- but what isn't nice is increasing the points where this stuff is decided at compile time. -Scott -- To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
