From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755795Ab2HNKN6 (ORCPT ); Tue, 14 Aug 2012 06:13:58 -0400 Received: from mx1.redhat.com ([209.132.183.28]:23419 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755718Ab2HNKN5 (ORCPT ); Tue, 14 Aug 2012 06:13:57 -0400 Date: Tue, 14 Aug 2012 13:13:53 +0300 From: Gleb Natapov To: "Michael S. Tsirkin" Cc: Alex Williamson , avi@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, jan.kiszka@siemens.com Subject: Re: [PATCH v7 2/2] kvm: KVM_EOIFD, an eventfd for EOIs Message-ID: <20120814101353.GA9738@redhat.com> References: <20120812074936.GA1421@redhat.com> <1344876495.4683.47.camel@ul30vt.home> <20120813165938.GA19139@redhat.com> <1344881845.4683.95.camel@ul30vt.home> <20120813194938.GB19139@redhat.com> <1344890905.4683.120.camel@ul30vt.home> <20120813215043.GA15639@redhat.com> <1344896532.4683.170.camel@ul30vt.home> <20120813225213.GB1931@redhat.com> <20120814101014.GF11194@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120814101014.GF11194@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 14, 2012 at 01:10:15PM +0300, Gleb Natapov wrote: > On Tue, Aug 14, 2012 at 01:52:13AM +0300, Michael S. Tsirkin wrote: > > > Using the EOI as a trigger to de-assert and potentially re-assert may be > > > a hack, but it's about as close as we can come to following the behavior > > > of hardware. > > > It's actually quite similar to an apic re-sampling inputs > > > except we don't have a physical line to read and see that it's still > > > asserted. We emulate this by de-asserting it and letting it re-assert > > > if necessary. The emulation to the guest isn't perfect, but it's a lot > > > closer than immediately de-asserting the pin. > > > I think the discussion > > > below describes why I do this versus something that might be even closer > > > to actual hardware. > > > > Sorry I don't understand what "quite similar" means. If deassert on ack > > is "closer" somehow show me some software that needs it. > > > This is incorrect question to ask. The correct one is "is there guest > visible effect" and the answer is yes. If guest reads ioapic irr before > eoi it will incorrectly read zero. Now when we know what is guest visible > effect we can think about whether we can live with it. But it looks like > we can't since this have more serious implications. If interrupt is masked > in ioapic during irq delivery interrupt will be never delivered after unmask. > Can be probably solved using mask notifiers, although I'd rather delete them than add new users. -- Gleb.