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

[Ben Hutchings <ben@decadent.org.uk>]

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On Wed, 2016-03-02 at 22:56 +0100, Radim Krčmář wrote:
> From: Radim Krčmář <rkrcmar@redhat.com>
> 
> commit 7dd0fdff145c5be7146d0ac06732ae3613412ac1 upstream.
> 
> 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.
> 
> The old policy breaks legacy NMI watchdogs, where PIT is used through
> virtual wire (LVT0): PIT never sends an interrupt before receiving EOI,
> thus a guest deadlock with disabled interrupts will stop NMIs.
> 
> Note that NMI doesn't do EOI, so PIT also had to send a normal interrupt
> through IOAPIC.  (KVM's PIT is deeply rotten and luckily not used much
> in modern systems.)
> 
> 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?

Ben.

-- 
Ben Hutchings
This sentence contradicts itself - no actually it doesn't.

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