From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:46648)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <avi@cloudius-systems.com>) id 1VlK2T-0008An-7n
	for qemu-devel@nongnu.org; Tue, 26 Nov 2013 09:55:04 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <avi@cloudius-systems.com>) id 1VlK2L-00071G-Tu
	for qemu-devel@nongnu.org; Tue, 26 Nov 2013 09:54:57 -0500
Received: from mail-ea0-f172.google.com ([209.85.215.172]:46088)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <avi@cloudius-systems.com>) id 1VlK2L-00071A-OE
	for qemu-devel@nongnu.org; Tue, 26 Nov 2013 09:54:49 -0500
Received: by mail-ea0-f172.google.com with SMTP id q10so3605204ead.17
	for <qemu-devel@nongnu.org>; Tue, 26 Nov 2013 06:54:48 -0800 (PST)
Message-ID: <5294B634.4050801@cloudius-systems.com>
Date: Tue, 26 Nov 2013 16:54:44 +0200
From: Avi Kivity <avi@cloudius-systems.com>
MIME-Version: 1.0
References: <D3E216785288A145B7BC975F83A2ED10448ADCCB@SZXEMA510-MBS.china.huawei.com>
	<52949847.6020908@redhat.com>
	<CAEbWaipAnmoa=gMbB1aNb=btU6LgYXhcnmQWHJ_89m4yvw6Dug@mail.gmail.com>
	<5294A68F.6060301@redhat.com>
	<CAF950W+-UiX6xv4vYmnxji9aWDU8ds2rnx7JugnbHQWJdCCD-Q@mail.gmail.com>
	<5294B461.5000405@redhat.com>
In-Reply-To: <5294B461.5000405@redhat.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [RFC] create a single workqueue for each vm to
 update vm irq routing table
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Huangweidong (C)" <weidong.huang@huawei.com>, KVM <kvm@vger.kernel.org>, Gleb Natapov <gleb@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>, "Zhanghaoyu (A)" <haoyu.zhang@huawei.com>, Luonengjun <luonengjun@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Zanghongyong <zanghongyong@huawei.com>, Avi Kivity <avi.kivity@gmail.com>, "Jinxin (F)" <jinxin712@huawei.com>

On 11/26/2013 04:46 PM, Paolo Bonzini wrote:
> Il 26/11/2013 15:36, Avi Kivity ha scritto:
>>      No, this would be exactly the same code that is running now:
>>
>>              mutex_lock(&kvm->irq_lock);
>>              old = kvm->irq_routing;
>>              kvm_irq_routing_update(kvm, new);
>>              mutex_unlock(&kvm->irq_lock);
>>
>>              synchronize_rcu();
>>              kfree(old);
>>              return 0;
>>
>>      Except that the kfree would run in the call_rcu kernel thread instead of
>>      the vcpu thread.  But the vcpus already see the new routing table after
>>      the rcu_assign_pointer that is in kvm_irq_routing_update.
>>
>> I understood the proposal was also to eliminate the synchronize_rcu(),
>> so while new interrupts would see the new routing table, interrupts
>> already in flight could pick up the old one.
> Isn't that always the case with RCU?  (See my answer above: "the vcpus
> already see the new routing table after the rcu_assign_pointer that is
> in kvm_irq_routing_update").

With synchronize_rcu(), you have the additional guarantee that any 
parallel accesses to the old routing table have completed.  Since we 
also trigger the irq from rcu context, you know that after 
synchronize_rcu() you won't get any interrupts to the old destination 
(see kvm_set_irq_inatomic()).

It's another question whether the hardware provides the same guarantee.

> If you eliminate the synchronize_rcu, new interrupts would see the new
> routing table, while interrupts already in flight will get a dangling
> pointer.

Sure, if you drop the synchronize_rcu(), you have to add call_rcu().