From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table Date: Thu, 28 Nov 2013 12:11:38 +0200 Message-ID: <20131128101138.GB5822@minantech.com> References: <5294A68F.6060301@redhat.com> <5294B461.5000405@redhat.com> <5294B634.4050801@cloudius-systems.com> <20131126150357.GA20352@redhat.com> <5294BC3B.6070902@redhat.com> <5297050E.6000700@redhat.com> <20131128091903.GA4609@kernel.org> <5297118C.3050104@cloudius-systems.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Paolo Bonzini , "Zhanghaoyu (A)" , Gleb Natapov , Avi Kivity , "Huangweidong (C)" , KVM , "Michael S. Tsirkin" , "Jinxin (F)" , Luonengjun , "qemu-devel@nongnu.org" , Zanghongyong To: Avi Kivity Return-path: Received: from mail-wg0-f42.google.com ([74.125.82.42]:38517 "EHLO mail-wg0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751112Ab3K1KLn (ORCPT ); Thu, 28 Nov 2013 05:11:43 -0500 Received: by mail-wg0-f42.google.com with SMTP id a1so639469wgh.5 for ; Thu, 28 Nov 2013 02:11:42 -0800 (PST) Content-Disposition: inline In-Reply-To: <5297118C.3050104@cloudius-systems.com> Sender: kvm-owner@vger.kernel.org List-ID: On Thu, Nov 28, 2013 at 11:49:00AM +0200, Avi Kivity wrote: > On 11/28/2013 11:19 AM, Gleb Natapov wrote: > >On Thu, Nov 28, 2013 at 09:55:42AM +0100, Paolo Bonzini wrote: > >>Il 28/11/2013 07:27, Zhanghaoyu (A) ha scritto: > >>>>>Without synchronize_rcu you could have > >>>>> > >>>>> VCPU writes to routing table > >>>>> e = entry from IRQ routing table > >>>>> kvm_irq_routing_update(kvm, new); > >>>>> VCPU resumes execution > >>>>> kvm_set_msi_irq(e, &irq); > >>>>> kvm_irq_delivery_to_apic_fast(); > >>>>> > >>>>>where the entry is stale but the VCPU has already resumed execution. > >>>>> > >>>If we use call_rcu()(Not consider the problem that Gleb pointed out temporarily) instead of synchronize_rcu(), should we still ensure this? > >>The problem is that we should ensure this, so using call_rcu is not > >>possible (even not considering the memory allocation problem). > >> > >Not changing current behaviour is certainly safer, but I am still not 100% > >convinced we have to ensure this. > > > >Suppose guest does: > > > >1: change msi interrupt by writing to pci register > >2: read the pci register to flush the write > >3: zero idt > > > >I am pretty certain that this code can get interrupt after step 2 on real HW, > >but I cannot tell if guest can rely on it to be delivered exactly after > >read instruction or it can be delayed by couple of instructions. Seems to me > >it would be fragile for an OS to depend on this behaviour. AFAIK Linux does not. > > > > Linux is safe, it does interrupt migration from within the interrupt > handler. If you do that before the device-specific EOI, you won't > get another interrupt until programming the MSI is complete. > > Is virtio safe? IIRC it can post multiple interrupts without guest acks. > > Using call_rcu() is a better solution than srcu IMO. Less code > changes, consistently faster. Why not fix userspace to use KVM_SIGNAL_MSI instead? -- Gleb.