From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:38071) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VlLC3-0006QG-0l for qemu-devel@nongnu.org; Tue, 26 Nov 2013 11:09:00 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1VlLBx-0006Or-0W for qemu-devel@nongnu.org; Tue, 26 Nov 2013 11:08:54 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38799) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VlLBw-0006Oj-Nb for qemu-devel@nongnu.org; Tue, 26 Nov 2013 11:08:48 -0500 Date: Tue, 26 Nov 2013 18:11:51 +0200 From: "Michael S. Tsirkin" Message-ID: <20131126161151.GC23007@redhat.com> References: <5294B461.5000405@redhat.com> <5294B634.4050801@cloudius-systems.com> <20131126150357.GA20352@redhat.com> <5294BC3B.6070902@redhat.com> <5294BD61.7080904@cloudius-systems.com> <5294BE22.7040105@redhat.com> <5294BFB7.2090202@cloudius-systems.com> <5294C53D.8060009@redhat.com> <5294C702.4070400@cloudius-systems.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5294C702.4070400@cloudius-systems.com> Subject: Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Avi Kivity Cc: "Huangweidong (C)" , KVM , Gleb Natapov , "Zhanghaoyu (A)" , Luonengjun , "qemu-devel@nongnu.org" , Zanghongyong , Avi Kivity , Paolo Bonzini , "Jinxin (F)" On Tue, Nov 26, 2013 at 06:06:26PM +0200, Avi Kivity wrote: > On 11/26/2013 05:58 PM, Paolo Bonzini wrote: > >Il 26/11/2013 16:35, Avi Kivity ha scritto: > >>>>>If we want to ensure, we need to use a different mechanism for > >>>>>synchronization than the global RCU. QRCU would work; readers are not > >>>>>wait-free but only if there is a concurrent synchronize_qrcu, which > >>>>>should be rare. > >>>>An alternative path is to convince ourselves that the hardware does not > >>>>provide the guarantees that the current code provides, and so we can > >>>>relax them. > >>>No, I think it's a reasonable guarantee to provide. > >>Why? > >Because IIUC the semantics may depend not just on the interrupt > >controller, but also on the specific PCI device. It seems safer to > >assume that at least one device/driver pair wants this to work. > > It's indeed safe, but I think there's a nice win to be had if we > drop the assumption. I'm not arguing with that, but a minor commoent below: > >(BTW, PCI memory writes are posted, but configuration writes are not). > > MSIs are configured via PCI memory writes. > > By itself, that doesn't buy us anything, since the guest could flush > the write via a read. But I think the fact that the interrupt > messages themselves are posted proves that it is safe. FYI, PCI read flushes the interrupt itself in, too. > The fact > that Linux does interrupt migration from within the interrupt > handler also shows that someone else believes that it is the only > safe place to do it. >