From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:58315)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <benh@kernel.crashing.org>) id 1UqAcO-0002HH-Aj
	for qemu-devel@nongnu.org; Fri, 21 Jun 2013 19:19:49 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <benh@kernel.crashing.org>) id 1UqAcN-0005R0-9K
	for qemu-devel@nongnu.org; Fri, 21 Jun 2013 19:19:48 -0400
Message-ID: <1371856771.3944.85.camel@pasglop>
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Sat, 22 Jun 2013 09:19:31 +1000
In-Reply-To: <1371856234.30572.140.camel@ul30vt.home>
References: <1371806575-19347-1-git-send-email-aik@ozlabs.ru>
	<1371806575-19347-3-git-send-email-aik@ozlabs.ru>
	<B63FF5F7-202D-488C-B91F-2578C58FB5FD@suse.de>
	<1371816181.3944.58.camel@pasglop>
	<4C36A400-2A38-458A-B849-8D8F6D49A566@suse.de>
	<1371816659.3944.63.camel@pasglop>
	<0E9E14EB-DD5F-4B7A-ACE5-6C9F0103324C@suse.de>
	<1371851687.3944.69.camel@pasglop>
	<B9E9CCF1-0FCA-42BA-A5FA-C3438CAE0AC9@suse.de>
	<1371853305.3944.83.camel@pasglop>
	<1371856234.30572.140.camel@ul30vt.home>
Content-Type: text/plain; charset="UTF-8"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH 2/3] KVM: add kvm_arch_irqchip_add_msi_route
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: Alex Williamson <alex.williamson@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, Alexey Kardashevskiy <aik@ozlabs.ru>, Alexander Graf <agraf@suse.de>, qemu-devel@nongnu.org, qemu-ppc@nongnu.org, David Gibson <david@gibson.dropbear.id.au>

On Fri, 2013-06-21 at 17:10 -0600, Alex Williamson wrote:

> MSI-X is rather ugly.  As you suggest, we trap accesses to the MSI-X
> table.  We don't know how many vectors the guest is going to use, so we
> incrementally add them by disabling and re-enabling with a new vector
> count.  The host decides what to put in the table, we don't care.  All
> interrupts bounce through the host and get to the guest via eventfd,
> either through qemu or directly through KVM irqfd.
> 
> If an in-use vector is modified, we write the new "MSI route" to KVM,
> the host doesn't need to care.  If a vector is masked, we free the host
> irq handler w/o modifying the vector configuration.  There's a comment
> in hw/misc/vfio.c that we could also just bounce masked vectors through
> qemu and let it drop it if we wanted to completely avoid toggling the
> host.  Linux currently does not have a usable interface for masking
> vectors at the device.  Thanks,

Right. It might also be handy to add what Willy's been talking about
for a while now, which is an interface to enable/setup individual MSI-X
at runtime.

Cheers,
Ben.