From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:34172) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gm1Pw-0005lX-TY for qemu-devel@nongnu.org; Tue, 22 Jan 2019 14:09:03 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gm1Pv-0000xx-Pi for qemu-devel@nongnu.org; Tue, 22 Jan 2019 14:09:00 -0500 Date: Tue, 22 Jan 2019 14:13:01 +0100 From: Cornelia Huck Message-ID: <20190122141301.5f69a9b0.cohuck@redhat.com> In-Reply-To: References: <20190122094143.8857-1-david@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v1] s390x/pci: Warn when adding PCI devices without the 'zpci' feature List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: David Hildenbrand Cc: Thomas Huth , qemu-devel@nongnu.org, qemu-s390x@nongnu.org, Collin Walling , Christian Borntraeger , Richard Henderson On Tue, 22 Jan 2019 11:06:46 +0100 David Hildenbrand wrote: > On 22.01.19 10:50, Thomas Huth wrote: > > On 2019-01-22 10:41, David Hildenbrand wrote: > >> We decided to always create the PCI host bridge, even if 'zpci' is not > >> enabled (due to migration compatibility). > > > > Couldn't we disable the host bridge for newer machine types, and just > > create it on the old ones for migration compatibility? I very dimly remember some problems with that approach. > > I think we can with a compat property. However I somewhat dislike that > the error/warning will then be "no bus" vs. "zpci CPU feature not > enabled". Somebody who has no idea about that will think he somehow has > to create a PCI bus on the QEMU comandline. Agreed, "zpci cpu feature not enabled" gives a much better clue. > > ... however > > > > >> This however right now allows > >> to add zPCI/PCI devices to a VM although the guest will never actually see > >> them, confusing people that are using a simple CPU model that has no > >> 'zpci' enabled - "Why isn't this working" (David Hildenbrand) > >> > >> Let's check for 'zpci' and at least print a warning that this will not > >> work as expected. We could also bail out, however that might break > >> existing QEMU commandlines. > >> > >> Signed-off-by: David Hildenbrand > >> --- > >> hw/s390x/s390-pci-bus.c | 5 +++++ > >> 1 file changed, 5 insertions(+) > >> > >> diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c > >> index b86a8bdcd4..e7d4f49611 100644 > >> --- a/hw/s390x/s390-pci-bus.c > >> +++ b/hw/s390x/s390-pci-bus.c > >> @@ -863,6 +863,11 @@ static void s390_pcihost_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev, > >> { > >> S390pciState *s = S390_PCI_HOST_BRIDGE(hotplug_dev); > >> > >> + if (!s390_has_feat(S390_FEAT_ZPCI)) { > >> + warn_report("Adding PCI or zPCI devices without the 'zpci' CPU feature." > >> + " The guest will not be able to see/use these devices."); > >> + } > > > > I think it would be better to bail out. The hotplug clearly can not work > > in this case, and the warn report might go unnoticed, so blocking the > > hotplug process is likely better to get the attention of the user. > > ... we could also create the bus but bail out here in case the compat > property strikes (a.k.a. new QEMO machine type). Now you confused me... why should failing be based on a compat property?