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

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Paul Mackerras's message of June 30, 2020 6:26 pm:
> 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.

Yep, thanks for catching it.

>> 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?

Just sent.

Thanks,
Nick