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

[Gleb Natapov <gleb@redhat.com>]

On Tue, Feb 05, 2013 at 01:26:42PM +0000, Zhang, Yang Z wrote:
> Gleb Natapov wrote on 2013-02-05:
> > 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.
> Sorry. I still cannot figure out which code is wrong. All the places call sync_pir_to_irr() are on target vcpu.
> Can you point out the code? Thanks.
> 
I am taking about vapic patches which are already in, not pir patches.

--
			Gleb.