From e2599de1e4856740fe2d482c3990a64924140545 Mon Sep 17 00:00:00 2001 From: =?utf8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?= Date: Wed, 2 Mar 2016 22:56:38 +0100 Subject: [PATCH] KVM: i8254: change PIT discard tick policy MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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. Reported-by: Yuki Shibuya Reviewed-by: Paolo Bonzini Signed-off-by: Radim Krčmář Signed-off-by: Paolo Bonzini [bwh: Backported to 3.2: - s/ps->reinject/ps->pit_timer.reinject/ - Adjust context] Signed-off-by: Ben Hutchings --- Reading git-format-patch failed