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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Mar 22, 2013 at 08:53:15AM +0200, Gleb Natapov wrote:
> 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.

See 4acd47cfea9c18134e0cbf915780892ef0ff433a on RHEL5, RHEL5 kernels before that
commit did not reinject.  Which means that all non-RHEL Linux guests
based on that upstream code also suffer from the same problem.

Also any other algorithm which uses LAPIC timers and compare that with
other clocks (such as NMI watchdog) are potentially vulnerable.

Can drop it, and then wait until someone complains (if so).

> Knowing that Linux tend to disable interrupt it is likely that it tries
> to detect and compensate for missing interrupt.

As said above, any algorithm which compares LAPIC timer interrupt with
another clock is vulnerable.