From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757992AbcBCQsp (ORCPT ); Wed, 3 Feb 2016 11:48:45 -0500 Received: from mail-wm0-f66.google.com ([74.125.82.66]:35823 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756337AbcBCQsm (ORCPT ); Wed, 3 Feb 2016 11:48:42 -0500 Subject: Re: [PATCH 3/4] KVM: x86: change PIT discard tick policy To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , linux-kernel@vger.kernel.org References: <1454516585-28491-1-git-send-email-rkrcmar@redhat.com> <1454516585-28491-4-git-send-email-rkrcmar@redhat.com> Cc: kvm@vger.kernel.org, Yuki Shibuya From: Paolo Bonzini Message-ID: <56B22F67.2000509@redhat.com> Date: Wed, 3 Feb 2016 17:48:39 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0 MIME-Version: 1.0 In-Reply-To: <1454516585-28491-4-git-send-email-rkrcmar@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > Signed-off-by: Radim Krčmář Haven't looked at the patch yet, but this is definitely how DISCARD is supposed to work. Paolo