[PATCH 3/3] powerpc/pseries: Add KVM guest doorbell restrictions

[Nicholas Piggin]

Excerpts from Paul Mackerras's message of June 30, 2020 12:27 pm:
> On Sun, Jun 28, 2020 at 01:04:28AM +1000, Nicholas Piggin wrote:
>> KVM guests have certain restrictions and performance quirks when
>> using doorbells. This patch tests for KVM environment in doorbell
>> setup, and optimises IPI performance:
>> 
>>  - PowerVM guests may now use doorbells even if they are secure.
>> 
>>  - KVM guests no longer use doorbells if XIVE is available.
> 
> It seems, from the fact that you completely remove
> kvm_para_available(), that you perhaps haven't tried building with
> CONFIG_KVM_GUEST=y.

It's still there and builds:

static inline int kvm_para_available(void)
{
        return IS_ENABLED(CONFIG_KVM_GUEST) && is_kvm_guest();
}

but...

> Somewhat confusingly, that option is not used or
> needed when building for a PAPR guest (i.e. the "pseries" platform)
> but is used on non-IBM platforms using the "epapr" hypervisor
> interface.

... is_kvm_guest() returns false on !PSERIES now. Not intended
to break EPAPR. I'm not sure of a good way to share this between
EPAPR and PSERIES, I might just make a copy of it but I'll see.

Thanks,
Nick