RE: [PATCH 2/2] x86, apicv: Add Posted Interrupt supporting

Zhang, Yang Z
2013-02-05T10:36:00Z

Gleb Natapov wrote on 2013-02-05:
> On Tue, Feb 05, 2013 at 05:57:14AM +0000, Zhang, Yang Z wrote:
>> Marcelo Tosatti wrote on 2013-02-05:
>>> On Mon, Feb 04, 2013 at 05:59:52PM -0200, Marcelo Tosatti wrote:
>>>> On Mon, Feb 04, 2013 at 07:13:01PM +0200, Gleb Natapov wrote:
>>>>> On Mon, Feb 04, 2013 at 12:43:45PM -0200, Marcelo Tosatti wrote:
>>>>>>>> Any example how software relies on such
>>> two-interrupts-queued-in-IRR/ISR behaviour?
>>>>>>> Don't know about guests, but KVM relies on it to detect interrupt
>>>>>>> coalescing. So if interrupt is set in IRR but not in PIR interrupt will
>>>>>>> not be reported as coalesced, but it will be coalesced during PIR->IRR
>>>>>>> merge.
>>>>>>
>>>>>> Yes, so:
>>>>>>
>>>>>> 1. IRR=1, ISR=0, PIR=0. Event: set_irq, coalesced=no.
>>>>>> 2. IRR=0, ISR=1, PIR=0. Event: IRR->ISR transfer.
>>>>>> 3. vcpu outside of guest mode.
>>>>>> 4. IRR=1, ISR=1, PIR=0. Event: set_irq, coalesced=no.
>>>>>> 5. vcpu enters guest mode.
>>>>>> 6. IRR=1, ISR=1, PIR=1. Event: set_irq, coalesced=no.
>>>>>> 7. HW transfers PIR into IRR.
>>>>>>
>>>>>> set_irq return value at 7 is incorrect, interrupt event was _not_
>>>>>> queued.
>>>>> Not sure I understand the flow of events in your description correctly. As I
>>>>> understand it at 4 set_irq() will return incorrect result. Basically
>>>>> when PIR is set to 1 while IRR has 1 for the vector the value of
>>>>> set_irq() will be incorrect.
>>>>
>>>> At 4 it has not been coalesced: it has been queued to IRR.
>>>> At 6 it has been coalesced: PIR bit merged into IRR bit.
>>>>
>>>>> Frankly I do not see how it can be fixed
>>>>> without any race with present HW PIR design.
>>>>
>>>> At kvm_accept_apic_interrupt, check IRR before setting PIR bit, if IRR
>>>> already set, don't set PIR.
>>>
>>> Or:
>>>
>>> apic_accept_interrupt() {
>>>
>>> 1. Read ORIG_PIR=PIR, ORIG_IRR=IRR.
>>> Never set IRR when HWAPIC enabled, even if outside of guest mode.
>>> 2. Set PIR and let HW or SW VM-entry transfer it to IRR.
>>> 3. set_irq return value: (ORIG_PIR or ORIG_IRR set).
>>> }
>>>
>>> Two or more concurrent set_irq can race with each other, though. Can
>>> either document the race or add a lock.
>> According the SDM, software should not touch the IRR when target vcpu is
> running. Instead, use locked way to access PIR. So your solution may wrong.
> Then your apicv patches are broken, because they do exactly that.
Which code is broken?

>> The only problem is the step 6, but at that point, there already an interrupt
> pending in IRR. This means the interrupt will be handled not lost. And even in real
> hardware, this case do exist. So I think it should not be a problem.
>>
> This is not the problem we are trying to fix. Sometimes we need to make
> sure that each interrupt device generates result in an interrupt handler
> invocation in a guest. If interrupt is coalesced (meaning it will not
> correspond to separate invocation of a guest interrupt handler) it needs
> to be re-injected. With PIR detection of such condition is broken.

Best regards,
Yang