[RFC PATCH 02/10] KVM: PPC: BOOK3S: PR: Emulate virtual timebase register

[Alexander Graf]

> Am 30.01.2014 um 06:49 schrieb Paul Mackerras <paulus at samba.org>:
> 
>> On Tue, Jan 28, 2014 at 10:14:07PM +0530, Aneesh Kumar K.V wrote:
>> virtual time base register is a per vm register and need to saved
>> and restored on vm exit and entry. Writing to VTB is not allowed
>> in the privileged mode.
> ...
> 
>> +#ifdef CONFIG_PPC_BOOK3S_64
>> +#define mfvtb()        ({unsigned long rval;                \
>> +            asm volatile("mfspr %0, %1" :            \
>> +                     "=r" (rval) : "i" (SPRN_VTB)); rval;})
> 
> The mfspr will be a no-op on anything before POWER8, meaning the
> result will be whatever value was in the destination GPR before the
> mfspr.  I suppose that may not matter if the result is only ever used
> when we're running on a POWER8 host, but I would feel more comfortable
> if we had explicit feature tests to make sure of that, rather than
> possibly doing computations with unpredictable values.
> 
> With your patch, a guest on a POWER7 or a PPC970 could do a read from
> VTB and get garbage -- first, there is nothing to stop userspace from
> requesting POWER8 emulation on an older machine, and secondly, even if
> the virtual machine is a PPC970 (say) you don't implement
> unimplemented SPR semantics for VTB (no-op if PR=0, illegal
> instruction interrupt if PR=1).
> 
> On the whole I think it is reasonable to reject an attempt to set the
> virtual PVR to a POWER8 PVR value if we are not running on a POWER8
> host, because emulating all the new POWER8 features in software
> (particularly transactional memory) would not be feasible.  Alex may
> disagree. :)

We don't have a good feature flag indicator that tells kvm what the guest cpu is capable of. So yes, I think it's reasonable to just not expose p8 registers on p8 for now.

In theory it's of course possible to emulate a lot of p8 features on pre-p8 hardware, but I'm not sure it's worth the effort. If anyone wants to spend the time to work on it I'd be happy to tale patches though ;)

Alex

> 
> Paul.