Re: [Qemu-devel] [PATCH] RFC: powerpc: add PVR compatibility check

[Alexander Graf]

On 04.11.2013, at 04:36, Alexey Kardashevskiy <address@hidden> wrote:

> If QEMU is started with KVM enabled and -cpu specified, and the CPU is not
> from the family which the host is running on, an error should be displayed
> so this the patch does.
> 
> Cc: Andreas Färber <address@hidden>
> Signed-off-by: Alexey Kardashevskiy <address@hidden>
> 
> ---
> 
> Is that correct to assume that the closest abstract class is a CPU family?
> It is most likely true but I want to double check :)

I don't think this is correct. KVM on POWER7 is compatible to run a 750 vcpu 
for example.

> Is there any nicer way of doing what the patch does?

The only instance that knows whether it's compatible or not is the kvm kernel 
module. Currently the only way we can check compatibility is through the "pvr" 
value that user space pushes into the kernel.

I see two ways we can enhance this. We could add checks to kvm's HV mode to 
make sure the guest vcpu type is compatible. Since the list of supported PVRs 
is quite small this might even be feasible.

The other thing that would be nice would be to transfer a full blob of 
capabilities into kvm that we can match for, similar to how cpuid on x86 works. 
That way we can then match host features with guest features and can check 
compatibility on a much more fine grained level.

The big benefit of the second approach is that when someone crazy enough comes 
in to implement e500mc on book3s kvm for example, he could do that simply by 
setting a few different capability bits. It would also make paired single 
selection more obvious for example. And we could limit Altivec access to only 
CPUs that have it rather than exposing it for all as we do today.


Alex