Re: [PATCH 10/10] KVM: PPC: Enable the PAPR CAP for Book3S

On Wed, Aug 10, 2011 at 09:59:41AM +0200, Alexander Graf wrote:

Am 10.08.2011 um 06:42 schrieb Paul Mackerras [off-list ref]:

quoted

On Tue, Aug 09, 2011 at 06:31:48PM +0200, Alexander Graf wrote:

quoted

Now that Book3S PV mode can also run PAPR guests, we can add a PAPR cap and
enable it for all Book3S targets. Enabling that CAP switches KVM into PAPR
mode.

Don't we want to enable it only for 64-bit hosts?  Trying to run a
PAPR guest on a 32-bit Book 3S host won't work very well, unless I am
missing something...

I agree that it doesn't make sense, but if anything we should
restrict it to 64-bit _guests_. you can also run 32-bit guests on
64-bit hosts.

I had a look in PAPR and I didn't find anything that says the
processor has to be 64-bit, so I guess a 32-bit PAPR guest is possible
in theory.  However, I don't think there are currently any 32-bit PAPR
operating systems that would use hcalls.

And so far, we don't have a single interface setting PVR and PAPR
mode at the same time, so you could still enable PAPR with a 64-bit
guest CPU and then switch to a 32-bit CPU.

It'd be a nightmare to check all configurations on every setter function.

Unless...

We could introduce a sanity check function that gets executed every
time we change PVR or enable PAPR. That could set a variable in the
vcpu struct to indicate that the config is ok. We could then check
that on vcpu_run.

It's probably not worth worrying about it.

The rest of the series looks very nice.

Regards,
Paul.