Re: [RFC PATCH] Fix split-irqchip vs interrupt injection window request.

[Paolo Bonzini <pbonzini@redhat.com>]

On 26/11/20 18:29, David Woodhouse wrote:
> On Thu, 2020-11-26 at 11:10 +0000, David Woodhouse wrote:
>>
>>> whether or not there's an IRQ in the
>>> LAPIC should be irrelevant when deciding to exit to userspace.  Note, the
>>> reinjection check covers vcpu->arch.interrupt.injected for the case where LAPIC
>>> is in userspace.
>>>
>>>          return kvm_arch_interrupt_allowed(vcpu) &&
>>>                 (!lapic_in_kernel(vcpu) || !kvm_cpu_has_extint(vcpu)) &&
>>>                 !kvm_event_needs_reinjection(vcpu) &&
>>>                 kvm_cpu_accept_dm_intr(vcpu);
>>> }
>>
>> Makes sense. I'm putting this version through some testing and will
>> post it later...
> 
> Hm, that survived enough test iterations to persuade me to post it, but
> then seems to have fallen over later. I'm reverting to the
> kvm_cpu_has_injectable_intr() version to leave that one running too and
> be sure it's gone in that.

!kvm_cpu_has_injectable_intr(vcpu) boils down (assuming no nested virt) to

         if (!lapic_in_kernel(v))
                 return !v->arch.interrupt.injected;

         if (kvm_cpu_has_extint(v))
                 return 0;

         return 1;

and Sean's proposal instead is the same indeed (the first "if" doesn't 
matter), so there may be more than one bug.

But it turns out that with some more inlining and Boolean algebra, we 
can actually figure out what the code does. :)  I had just finished 
writing a looong review of your patch starting from that idea, so I'll 
post it.

Paolo