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.
> 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.
--
Gleb.
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