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

[Gleb Natapov <gleb@redhat.com>]

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.

> >> 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

--
			Gleb.