Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy

[Paolo Bonzini]

On 17/02/2016 20:14, Radim KrÄmÃÅ wrote:
> Discard policy uses ack_notifiers to prevent injection of PIT interrupts
> before EOI from the last one.
> 
> This patch changes the policy to always try to deliver the interrupt,
> which makes a difference when its vector is in ISR.
> Old implementation would drop the interrupt, but proposed one injects to
> IRR, like real hardware would.

This seems like what libvirt calls the "merge" policy:

    Merge the missed tick(s) into one tick and inject. The guest time
    may be delayed, depending on how the OS reacts to the merging of
    ticks

where the merged tick is the one placed into IRR.  Unlike discard,
"merge" can starve the guest through an interrupt storm.

Rik, I think you originally worked on the missed tick policies in Xen.
Is the above correct?

If so, do you recall what would be the reason to use the merge policy
instead of the discard policy?

Paolo