Re: [PATCH v2 6/8] arm/arm64: KVM: Add forwarded physical interrupts documentation

[Eric Auger <eric.auger@linaro.org>]

Hi Marc,
On 09/07/2015 07:50 PM, Marc Zyngier wrote:
> On 07/09/15 17:45, Eric Auger wrote:
>> Hi Christoffer,
>> On 09/04/2015 09:40 PM, Christoffer Dall wrote:
>>> Forwarded physical interrupts on arm/arm64 is a tricky concept and the
>>> way we deal with them is not apparently easy to understand by reading
>>> various specs.
>>>
>>> Therefore, add a proper documentation file explaining the flow and
>>> rationale of the behavior of the vgic.
>>>
>>> Some of this text was contributed by Marc Zyngier and edited by me.
>>> Omissions and errors are all mine.
>>>
>>> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
>>> ---
>>>  Documentation/virtual/kvm/arm/vgic-mapped-irqs.txt | 181 +++++++++++++++++++++
>>>  1 file changed, 181 insertions(+)
>>>  create mode 100644 Documentation/virtual/kvm/arm/vgic-mapped-irqs.txt
>>>
>>> diff --git a/Documentation/virtual/kvm/arm/vgic-mapped-irqs.txt b/Documentation/virtual/kvm/arm/vgic-mapped-irqs.txt
>>> new file mode 100644
>>> index 0000000..24b6f28
>>> --- /dev/null
>>> +++ b/Documentation/virtual/kvm/arm/vgic-mapped-irqs.txt
>>> @@ -0,0 +1,181 @@
>>> +KVM/ARM VGIC Forwarded Physical Interrupts
>>> +==========================================
> 
> [...]
> 
>>> +1.  KVM runs the VCPU
>>> +2.  The guest programs the time to fire in T+100
>>> +4.  At T+100 the timer fires and a physical IRQ causes the VM to exit
>>> +5.  With interrupts disabled on the CPU, KVM looks at the timer state
>>> +    and injects a forwarded physical interrupt because it concludes the
>>> +    timer has expired.
>> I don't get how we can trap without the virtual timer PPI handler being
>> entered on host side. Please can you elaborate on this?
> 
> On VM exit, we disable the virtual timer (see the code in
> hyp.S::save_timer_state where we clear the enable bit). We still perform
> the exit, but the cause for exit is now gone, and the handler will never
> fire.
OK thanks for the clarification

Eric
> 
> Thanks,
> 
> 	M.
>