From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table Date: Thu, 28 Nov 2013 11:49:00 +0200 Message-ID: <5297118C.3050104@cloudius-systems.com> References: <52949847.6020908@redhat.com> <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> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: "Zhanghaoyu (A)" , Gleb Natapov , Avi Kivity , "Huangweidong (C)" , KVM , "Michael S. Tsirkin" , "Jinxin (F)" , Luonengjun , "qemu-devel@nongnu.org" , Zanghongyong To: Gleb Natapov , Paolo Bonzini Return-path: Received: from mail-bk0-f52.google.com ([209.85.214.52]:41896 "EHLO mail-bk0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750935Ab3K1JtG (ORCPT ); Thu, 28 Nov 2013 04:49:06 -0500 Received: by mail-bk0-f52.google.com with SMTP id u14so3663711bkz.39 for ; Thu, 28 Nov 2013 01:49:04 -0800 (PST) In-Reply-To: <20131128091903.GA4609@kernel.org> Sender: kvm-owner@vger.kernel.org List-ID: 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.