Re: [PATCH 2/5] KVM: x86: do not scan IRR twice on APICv vmentry

[Radim KrÄmÃÅ]

2016-11-03 17:00+0100, Paolo Bonzini:
> On 03/11/2016 16:03, Radim KrÄmÃÅ wrote:
>> 2016-11-03 14:30+0100, Paolo Bonzini:
>>> On 26/10/2016 21:59, Radim KrÄmÃÅ wrote:
>>>> 2016-10-14 20:21+0200, Paolo Bonzini:
>>>>> Calling apic_find_highest_irr results in IRR being scanned twice,
>>>>> once in vmx_sync_pir_from_irr and once in apic_search_irr.  Change
>>>>> sync_pir_from_irr to do the RVI write and kvm_apic_update_irr to
>>>>> compute the new RVI on the fly.
>>>>>
>>>>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>>>>> ---
>>>>
>>>> Reviewed-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>>>
>>> Nope, this breaks nested VMX exit on external interrupt.  For now I'm
>>> testing only patch 1 and will push that one only to kvm/next.

Which hypervisor is being nested?

>> Hm, does it also happen with this change?
> 
> Probably not but I wanted to understand why. :)

Yeah, I'm also looking forward to knowing the funny coincidence.

I think a bug is likely for hypervisors that don't enable
PIN_BASED_EXT_INTR_MASK.  The bug would trigger when
kvm_cpu_has_interrupt() in vmx_check_nested_events() in
kvm_arch_vcpu_runnable() queues the interrupt ...
but I didn't see how this would have caused a problem. :)