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

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Message-ID: <18e681d1-640f-8c56-dc05-acc7ca34cd45@redhat.com>
Date:   Thu, 3 Nov 2016 19:18:00 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        yang.zhang.wz@...il.com, feng.wu@...el.com, mst@...hat.com
Subject: Re: [PATCH 2/5] KVM: x86: do not scan IRR twice on APICv vmentry



On 03/11/2016 19:07, Radim Krčmář wrote:
> 2016-11-03 17:00+0100, Paolo Bonzini:
>> On 03/11/2016 16:03, Radim Krčmář wrote:
>>> 2016-11-03 14:30+0100, Paolo Bonzini:
>>>> On 26/10/2016 21:59, Radim Krčmář 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@...hat.com>
>>>>>> ---
>>>>>
>>>>> Reviewed-by: Radim Krčmář <rkrcmar@...hat.com>
>>>>
>>>> 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?

vmx.flat. :)

> 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. :)

Ironically, _not_ enabling PIN_BASED_EXT_INTR_MASK and not using HALT
activity state is the only case that passes of the four that vmx.flat tests.

Paolo