Re: [Qemu-devel] [RFC PATCH v3] powerpc: add PVR mask support

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2013-08-15 at 19:28 -0500, Anthony Liguori wrote:
> On Thu, Aug 15, 2013 at 7:20 PM, Benjamin Herrenschmidt
> <benh@kernel.crashing.org> wrote:
> > On Thu, 2013-08-15 at 16:47 +0200, Andreas Färber wrote:
> >> comparing values for closest match. So that if you have a v2.4 and QEMU
> >> knows v2.1 and v2.3 we take v2.3 and fill in the v2.4 PVR.
> >
> > Another thing to keep in mind is that we will want eventually to support
> > POWER7 compatibility more on POWER8 with HV KVM. I am not certain what
> > the "right" way to do it via qemu command line is, whether we would
> > have a -cpu entry (-cpu POWER7_COMPAT ?) or such...
> 
> But this doesn't change the hardware PVR value right, just the virtual
> PVR that's in the device tree?  

Right.

> Maybe you need to change some state of the virtual VCPU but from a QEMU
> point of view, it's still a POWER8 VCPU (it's just in power7 compat mode).

Sure, as long as TCG does the right thing (some instructions must be
forbidden etc...). That's more what I had in mind...

> > Additionally, the trick here is that qemu must be able to change its model
> > at runtime (a reset is permitted). This is how PAPR defines the reconfiguration
> > reboot (for that and other things).
> 
> I don't think the model changes.  It's just a flag in the power8 vcpu state.
> 
> No different IMHO between jumping from real mode to protected mode to
> long mode on x86.

Ok.

Cheers,
Ben.

> Regards,
> 
> Anthony Liguori
> 
> > IE. The guest kernel will call FW early on, while still operating under
> > OFW (from prom_init) indicating what it supports, and if that doesn't include
> > P8, we need to reconfigure the CPU model to be P7 compat (we are allowed to
> > reboot and reload the same kernel, which is generally what pHyp does, but
> > we'd like to try avoiding it as much as possible).
> >
> > Cheers,
> > Ben.
> >
> >
> >