Re: [PATCH 4.5 007/238] KVM: i8254: change PIT discard tick policy

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-04-11 18:21-0700, Greg Kroah-Hartman:
> On Tue, Apr 12, 2016 at 12:23:35AM +0100, Ben Hutchings wrote:
> > On Wed, 2016-03-02 at 22:56 +0100, Radim Krčmář wrote:
>> > Even though there is a chance of regressions, I think we can fix the
>> > LVT0 NMI bug without introducing a new tick policy.
>> [...]
>> 
>> Given the 'chance of regressions', should we let this sit in mainline
>> longer before including it in stable updates?
> 
> Hm, good point, Radim, what do you think, is this good to go to stable
> now?  This has been in since 4.6-rc1, so it's been a few weeks with
> people running it already...

I think it is good to go.  No reasonable workload should regress and the
fixed use-case is common on old linux guest.

This patch makes a difference if the guest doesn't EOI in PIT interrupts
before the next one arrives.  PIT would have been unreliable in that
situation, so all worloads that that could regress have likely been
avoided.  Changes to NMI injection would need even crazier guest to
regress.

(This patch always injects NMI from PIT and shrinks a window where a
 maskable PIT interrupt is discarded.  Previously, next interrupt would
 have been discarded as long as the last one is in IRR or ISR.  Only IRR
 is considered after this patch, so PIT interrupts are more likely to
 chain.)