Re: [PATCH] KVM: x86: Avoid busy loops over uninjectable pending APIC timers

[Gleb Natapov <gleb@redhat.com>]

On Thu, Mar 21, 2013 at 08:06:41PM -0300, Marcelo Tosatti wrote:
> On Thu, Mar 21, 2013 at 11:13:39PM +0200, Gleb Natapov wrote:
> > On Thu, Mar 21, 2013 at 05:51:50PM -0300, Marcelo Tosatti wrote:
> > > > > > But current PI patches do break them, thats my point. So we either
> > > > > > need to revise them again, or drop LAPIC timer reinjection. Making
> > > > > > apic_accept_irq semantics "it returns coalescing info, but only sometimes"
> > > > > > is dubious though.
> > > > > We may rollback to the initial idea: test both irr and pir to get coalescing info. In this case, inject LAPIC timer always in vcpu context. So apic_accept_irq() will return right coalescing info.
> > > > > Also, we need to add comments to tell caller, apic_accept_irq() can ensure the return value is correct only when caller is in target vcpu context.
> > > > > 
> > > > We cannot touch irr while vcpu is in non-root operation, so we will have
> > > > to pass flag to apic_accept_irq() to let it know that it is called
> > > > synchronously. While all this is possible I want to know which guests
> > > > exactly will we break if we will not track interrupt coalescing for
> > > > lapic timer. If only 2.0 smp kernels will break we can probably drop it.
> > > 
> > > RHEL4 / RHEL5 guests.
> > RHEL5 has kvmclock no? We should not break RHEL4 though.
> 
> kvmclock provides no timer interrupt... either LAPIC or PIT must be used
> with kvmclock.
I am confused now. If LAPIC is not used for wallclock time keeping, but
only for scheduling the reinjection is actually harmful. Reinjecting the
interrupt will cause needles task rescheduling. So the question is if
there is a Linux kernel that uses LAPIC for wallclock time keeping and
relies on accurate number of injected interrupts to not time drift.
Knowing that Linux tend to disable interrupt it is likely that it tries
to detect and compensate for missing interrupt.

--
			Gleb.