linux-kernel - Re: [PATCH 3/4] KVM: x86: change PIT discard tick policy

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Date:	Wed, 3 Feb 2016 17:48:39 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>,
	linux-kernel@...r.kernel.org
Cc:	kvm@...r.kernel.org, Yuki Shibuya <shibuya.yk@...s.nec.co.jp>
Subject: Re: [PATCH 3/4] KVM: x86: change PIT discard tick policy



On 03/02/2016 17:23, Radim Krčmář 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.
> 
> 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.
> 
> Reported-by: Yuki Shibuya <shibuya.yk@...s.nec.co.jp>
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>

Haven't looked at the patch yet, but this is definitely how DISCARD is
supposed to work.

Paolo