From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:45163) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gm1o5-0007kv-Rs for qemu-devel@nongnu.org; Tue, 22 Jan 2019 14:34:01 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gm1cO-0006b2-2i for qemu-devel@nongnu.org; Tue, 22 Jan 2019 14:21:53 -0500 References: <20190122094143.8857-1-david@redhat.com> <20190122141301.5f69a9b0.cohuck@redhat.com> <58639d62-019a-2d87-37e4-10523947e721@redhat.com> <20190122142311.5c7d3947.cohuck@redhat.com> From: David Hildenbrand Message-ID: <49b4c1cf-1dfb-c0c9-064d-48b71d35e8c8@redhat.com> Date: Tue, 22 Jan 2019 14:25:07 +0100 MIME-Version: 1.0 In-Reply-To: <20190122142311.5c7d3947.cohuck@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US 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: Cornelia Huck Cc: Thomas Huth , qemu-devel@nongnu.org, qemu-s390x@nongnu.org, Collin Walling , Christian Borntraeger , Richard Henderson On 22.01.19 14:23, Cornelia Huck wrote: > On Tue, 22 Jan 2019 14:20:27 +0100 > David Hildenbrand wrote: > >> On 22.01.19 14:13, Cornelia Huck wrote: >>> 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? >>> >> >> Otherwise, a QEMU comandline that used to work (which could be created >> by libvirt) would now fail. Are we ok with that? >> > > I think we should not fail at all in that case, then. Or only for > hotplug, not for coldplug. > We could fail on hotplug and warn on coldplug. This would keep existing setups running. -- Thanks, David / dhildenb