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?
>
I see a couple of possible solutions:
1. Do what Avi said. Make KVM_IRQ_LINE_STATUS be synchronous. Cons:
current QEMU uses KVM_IRQ_LINE_STATUS always and it means that it
will be slow on newer kernels
2. Make KVM_IRQ_LINE_STATUS report coalescing only when vcpu is not
running during injection. This assumes that if vcpu is running and does
not process interrupt it is guest fault and the same can happen on real
HW too. Coalescing when vcpu is not running though is the result of CPU
overcommit and should be reported. Cons interface definition is kind of
murky.
3. Do not report KVM_IRQ_LINE_STATUS capability and move RTC to use EOI
notifiers for interrupt reinjection. This requires us to add interface
for reporting EOI to userspace. This is not in the scope of this
patchset. Cons: need to introduce new interface (and the one that will
not work on AMD BTW)
Other ideas?
--
Gleb.
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