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

[Gleb Natapov <gleb@redhat.com>]

On Tue, Feb 05, 2013 at 10:58:28AM +0000, Zhang, Yang Z wrote:
> Gleb Natapov wrote on 2013-02-05:
> > On Tue, Feb 05, 2013 at 10:35:55AM +0000, Zhang, Yang Z wrote:
> >> 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 one that updates IRR directly on the apic page.
> No, all the updates are ensuring the target vcpu is not running. So it's safe to touch IRR.
> 
Not at all. Read the code.

--
			Gleb.