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

[Paul Mackerras <paulus@ozlabs.org>]

On Tue, Jun 30, 2020 at 03:35:08PM +1000, Nicholas Piggin wrote:
> 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:

OK, good, I missed that.

> 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.

And therefore kvm_para_available() returns false on all the platforms
where the code that depends on it could actually be used.

It's not correct to assume that !PSERIES means not a KVM guest.

> 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.

OK, so you're doing a new version?

Regards,
Paul.