Re: [PATCH v4 2/2] KVM: VMX: Add Posted Interrupt supporting

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Feb 25, 2013 at 11:13:25AM +0000, Zhang, Yang Z wrote:
> Gleb Natapov wrote on 2013-02-25:
> > On Mon, Feb 25, 2013 at 11:04:25AM +0000, Zhang, Yang Z wrote:
> >> Gleb Natapov wrote on 2013-02-25:
> >>> On Mon, Feb 25, 2013 at 08:42:52AM +0000, Zhang, Yang Z wrote:
> >>>> Avi Kivity wrote on 2013-02-25:
> >>>>> I didn't really follow, but is the root cause the need to keep track
> >>>>> of interrupt coalescing?  If so we can recommend that users use
> >>>>> KVM_IRQ_LINE when coalescing is unneeded, and move interrupt
> >>>>> injection with irq coalescing support to vcpu context.
> >>>> So we can hide the capability KVM_CAP_IRQ_INJECT_STATUS when posted
> >>> interrupt is enabled to force users doesn't to use
> >>> KVM_IRQ_LINE_STATUS. Does this acceptable?
> >>>> 
> >>>> The only case in KVM that need to know the interrupt injection status is
> > vlapic
> >>> timer. But since vlapic timer and vcpu are always in same pcpu, so there is no
> >>> problem.
> >>>> 
> >>> Not really. The primary user of this interface is RTC interrupt
> >>> re-injection for Windows guests.
> >> So without KVM_IRQ_LINE_STATUS capability, RTC cannot work well?
> >> 
> > Windows guests may experience timedrift under CPU overcommit scenario.
> Ok, I see. Seems we are stuck. :(
> Do you have any suggestion to solve or workaround current problem?

Depend on knowledge about atomicity (item 5 IIRC) of the sequence 
in the manual.