From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH v3 5/5] KVM: Allow host IRQ sharing for passed-through
 PCI 2.3 devices
Date: Wed, 3 Nov 2010 11:10:45 +0200
Message-ID: <20101103091045.GH6772@redhat.com>
References: <cover.1288771873.git.jan.kiszka@web.de>
 <628f014fb1efb8e2208db03d13198ba301a3a34c.1288771873.git.jan.kiszka@web.de>
 <20101103082921.GD6772@redhat.com>
 <4CD123B8.8080607@web.de>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Avi Kivity <avi@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>,
	kvm <kvm@vger.kernel.org>,
	Alex Williamson <alex.williamson@redhat.com>,
	Jan Kiszka <jan.kiszka@siemens.com>
To: Jan Kiszka <jan.kiszka@web.de>
Return-path: <kvm-owner@vger.kernel.org>
Received: from mx1.redhat.com ([209.132.183.28]:63805 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1753685Ab0KCJKs (ORCPT <rfc822;kvm@vger.kernel.org>);
	Wed, 3 Nov 2010 05:10:48 -0400
Content-Disposition: inline
In-Reply-To: <4CD123B8.8080607@web.de>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

On Wed, Nov 03, 2010 at 09:56:24AM +0100, Jan Kiszka wrote:
> > Hmm, this does an extra config read on each interrupt (another one is in
> > pci_2_3_irq_unmask).  These reads are pretty expensive... I do realize
> > locking becomes ugly, though. Maybe my idea to avoid set level to 0
> > was silly? Thoughts?
> 
> Well, reading twice is the price to pay here, putting kvm_set_irq under
> spin_lock_irq again is a no-go. From that POV, the previous version was
> probably the cheapest: no extra efforts in the common case, but still
> avoiding reassertion via the host IRQ handler whenever possible.
> 
> Jan
> 

Sigh. I guess so. Any chance of a benchmark to let us figure this out?

-- 
MST