From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from [140.186.70.92] (port=54446 helo=eggs.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1Pu51n-0002Bs-N1
	for qemu-devel@nongnu.org; Mon, 28 Feb 2011 10:28:52 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <stefanha@gmail.com>) id 1Pu51m-0002Ze-R8
	for qemu-devel@nongnu.org; Mon, 28 Feb 2011 10:28:51 -0500
Received: from mail-yw0-f45.google.com ([209.85.213.45]:38343)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <stefanha@gmail.com>) id 1Pu51m-0002ZZ-OZ
	for qemu-devel@nongnu.org; Mon, 28 Feb 2011 10:28:50 -0500
Received: by ywl41 with SMTP id 41so1602031ywl.4
	for <qemu-devel@nongnu.org>; Mon, 28 Feb 2011 07:28:50 -0800 (PST)
MIME-Version: 1.0
In-Reply-To: <0948831a888a7047d9e1ce945197a0210c922b1b.1298891447.git.amit.shah@redhat.com>
References: <cover.1298891447.git.amit.shah@redhat.com>
	<0948831a888a7047d9e1ce945197a0210c922b1b.1298891447.git.amit.shah@redhat.com>
Date: Mon, 28 Feb 2011 15:28:49 +0000
Message-ID: <AANLkTinrtaktYPPhcrz_yCvA8vEOHbND8CiO8h+PH_vD@mail.gmail.com>
Subject: Re: [Qemu-devel] [PATCH (resend,
	rebase) 3/3] virtio-serial: Enable ioeventfd
From: Stefan Hajnoczi <stefanha@gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/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: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Amit Shah <amit.shah@redhat.com>
Cc: qemu list <qemu-devel@nongnu.org>

On Mon, Feb 28, 2011 at 11:12 AM, Amit Shah <amit.shah@redhat.com> wrote:
> Enable ioeventfd for virtio-serial devices by default. =A0Commit
> 25db9ebe15125deb32958c6df74996f745edf1f9 lists the benefits of using
> ioeventfd.
>
> Copying a file from guest to host over a virtio-serial channel didn't
> show much difference in time or io_exit rate.

The cost of enabling ioeventfd is one eventfd file descriptor and KVM
in-kernel device slot per virtqueue.  The current maximum number per
VM is 200, this is a kernel limit in
include/linux/kvm_host.h:NR_IOBUS_DEVS.

Do you really want to use ioeventfd for virtio-serial?  Perhaps this
is more useful for high-frequency device interfaces.

Stefan