From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=38351 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PgI7j-0007MC-Ni for qemu-devel@nongnu.org; Fri, 21 Jan 2011 09:38:01 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PgI7i-0001aU-B0 for qemu-devel@nongnu.org; Fri, 21 Jan 2011 09:37:59 -0500 Received: from mail-qw0-f45.google.com ([209.85.216.45]:40083) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PgI7i-0001aM-7X for qemu-devel@nongnu.org; Fri, 21 Jan 2011 09:37:58 -0500 Received: by qwk4 with SMTP id 4so1846434qwk.4 for ; Fri, 21 Jan 2011 06:37:57 -0800 (PST) Message-ID: <4D399A44.5060405@codemonkey.ws> Date: Fri, 21 Jan 2011 08:37:56 -0600 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] [PATCH] vhost: force vhost off for non-MSI guests References: <20110120153521.GA24357@redhat.com> <4D38583D.9010602@codemonkey.ws> <20110120160718.GA24652@redhat.com> <4D38D208.4090403@codemonkey.ws> <20110121094827.GA26070@redhat.com> In-Reply-To: <20110121094827.GA26070@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: kvm@vger.kernel.org, Juan Quintela , Jes.Sorensen@redhat.com, jasowang@redhat.com, qemu-devel@nongnu.org, Alex Williamson On 01/21/2011 03:48 AM, Michael S. Tsirkin wrote: > On Thu, Jan 20, 2011 at 06:23:36PM -0600, Anthony Liguori wrote: > >> On 01/20/2011 10:07 AM, Michael S. Tsirkin wrote: >> >>> On Thu, Jan 20, 2011 at 09:43:57AM -0600, Anthony Liguori wrote: >>> >>>> On 01/20/2011 09:35 AM, Michael S. Tsirkin wrote: >>>> >>>>> When MSI is off, each interrupt needs to be bounced through the io >>>>> thread when it's set/cleared, so vhost-net causes more context switches and >>>>> higher CPU utilization than userspace virtio which handles networking in >>>>> the same thread. >>>>> >>>>> We'll need to fix this by adding level irq support in kvm irqfd, >>>>> for now disable vhost-net in these configurations. >>>>> >>>>> Signed-off-by: Michael S. Tsirkin >>>>> >>>> I actually think this should be a terminal error. The user asks for >>>> vhost-net, if we cannot enable it, we should exit. >>>> >>>> Or we should warn the user that they should expect bad performance. >>>> Silently doing something that the user has explicitly asked us not >>>> to do is not a good behavior. >>>> >>>> Regards, >>>> >>>> Anthony Liguori >>>> >>> The issue is that user has no control of the guest, and can not know >>> whether the guest enables MSI. So what you ask for will just make >>> some guests fail, and others fail sometimes. >>> The user also has no way to know that version X of kvm does not expose a >>> way to inject level interrupts with irqfd. >>> >>> We could have *another* flag that says "use vhost where it helps" but >>> then I think this is what everyone wants to do, anyway, and libvirt >>> already sets vhost=on so I prefer redefining the meaning of an existing >>> flag. >>> >> In the very least, there needs to be a vhost=force. >> Having some sort of friendly default policy is fine but we need to >> provide a mechanism for a user to have the final say. If you want >> to redefine vhost=on to really mean, use the friendly default, >> that's fine by me, but only if the vhost=force option exists. >> > OK, I will add that, probably as a separate flag as vhost > is a boolean. This will get worse performance but it will be what the > user asked for. > > >> I actually would think libvirt would want to use vhost=force. >> Debugging with vhost=on is going to be a royal pain in the ass if a >> user reports bad performance. Given the libvirt XML, you can't >> actually tell from the guest and the XML whether or not vhost was >> actually in use or not. >> > Yes you can: check MSI enabled in the guest, if it is - > check vhost enabled in the XML. Not that bad at all, is it? > Until you automatically detect level triggered interrupt support for irqfd. This means it's also dependent on a kernel feature too. Is there any way to tell in QEMU that vhost was silently disabled? Regards, Anthony Liguori > >> Regards, >> >> Anthony Liguori >> > We get worse performance without MSI anyway, how is this different? > > >>> Maybe this is best handled by a documentation update? >>> >>> We always said: >>> " use vhost=on to enable experimental in kernel accelerator\n" >>> >>> note 'enable' not 'require'. This is similar to how we specify >>> nvectors : you can not make guest use the feature. >>> >>> How about this: >>> >>> diff --git a/qemu-options.hx b/qemu-options.hx >>> index 898561d..3c937c1 100644 >>> --- a/qemu-options.hx >>> +++ b/qemu-options.hx >>> @@ -1061,6 +1061,7 @@ DEF("net", HAS_ARG, QEMU_OPTION_net, >>> " use vnet_hdr=off to avoid enabling the IFF_VNET_HDR tap flag\n" >>> " use vnet_hdr=on to make the lack of IFF_VNET_HDR support an error condition\n" >>> " use vhost=on to enable experimental in kernel accelerator\n" >>> + " (note: vhost=on has no effect unless guest uses MSI-X)\n" >>> " use 'vhostfd=h' to connect to an already opened vhost net device\n" >>> #endif >>> "-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n" >>> >>> >>> >