From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45147) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cRcfZ-0004Jn-KY for qemu-devel@nongnu.org; Thu, 12 Jan 2017 05:31:47 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cRcfW-0002DN-Iw for qemu-devel@nongnu.org; Thu, 12 Jan 2017 05:31:45 -0500 Message-ID: <1484217095.7948.1.camel@redhat.com> From: Andrea Bolognani Date: Thu, 12 Jan 2017 11:31:35 +0100 In-Reply-To: <20170108234621.GB12515@umbus.fritz.box> References: <20170105054618.GA12106@umbus.fritz.box> <1483724069.4199.80.camel@redhat.com> <20170108234621.GB12515@umbus.fritz.box> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] Proposal PCI/PCIe device placement on PAPR guests List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: David Gibson Cc: thuth@redhat.com, lvivier@redhat.com, benh@kernel.crashing.org, marcel@redhat.com, aik@ozlabs.ru, groug@kaod.org, ehabkost@redhat.com, mdroth@linux.vnet.ibm.com, libvir-list@redhat.com, qemu-devel@nongnu.org, qemu-ppc@nongnu.org, laine@redhat.com On Mon, 2017-01-09 at 10:46 +1100, David Gibson wrote: > > >=C2=A0=C2=A0=C2=A0=C2=A0* To allow for hotplugged devices, libvirt s= hould also add a number > > >=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0of additional, empty vPHBs (the = PAPR spec allows for hotplug of > > >=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0PHBs, but this is not yet implem= ented in qemu). > >=C2=A0 > > "A number" here will have to mean "one", same number of > > empty PCIe Root Ports libvirt will add to a newly-defined > > q35 guest. >=C2=A0 > Umm.. why? Because some applications using libvirt would inevitably start relying on the fact that such spare PHBs are available, locking us into providing at least the same number forever. In other words, increasing the amount at a later time is always possible, but decreasing it isn't. We did the same when we started automatically adding PCIe Root Ports to q35 machines. The rationale is that having a single spare hotpluggable slot is extremely convenient for basic usage, eg. a simple guest created by someone who's not necessarily very familiar with virtualization; on the other hand, if you are actually deploying in production you ought to conduct proper capacity planning and figure out in advance how many devices you're likely to need to hotplug throughout the guest's life. Of course this all will be moot once we can hotplug PHBs :) --=C2=A0 Andrea Bolognani / Red Hat / Virtualization