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Violators will be prosecuted for from ; Wed, 14 Dec 2016 13:12:22 -0000 Date: Wed, 14 Dec 2016 14:12:10 +0100 From: Cornelia Huck In-Reply-To: <20161214125237.20850-1-maxime.coquelin@redhat.com> References: <20161214125237.20850-1-maxime.coquelin@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Message-Id: <20161214141210.22d12962.cornelia.huck@de.ibm.com> Subject: Re: [Qemu-devel] [PATCH] virtio-pci: Fix cross-version migration with older machines List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Maxime Coquelin Cc: mdroth@linux.vnet.ibm.com, stefanha@redhat.com, qemu-devel@nongnu.org, mst@redhat.com, marcel@redhat.com, "Dr . David Alan Gilbert" On Wed, 14 Dec 2016 13:52:37 +0100 Maxime Coquelin wrote: > This patch fixes a cross-version migration regression introduced > by commit d1b4259f ("virtio-bus: Plug devices after features are > negotiated"). > > The problem is encountered when host's vhost backend does not support > VIRTIO_F_VERSION_1, and migration is initiated from a v2.7 or prior > machine with virtio-pci modern capabilities enabled to a v2.8 machine. Wait, machine versions or qemu versions? > > In this case, modern capabilities get exposed to the guest by the source, > whereas the target will detect version 1 is not supported so will only > expose legacy capabilities. > > The problem is fixed by introducing a new "x-modern-broken" property, > which is set in v2.7 and prior compatibility modes. Doing this, v2.7 > machine keeps its broken behaviour (enabling modern while version is > not supported), and newer machines will behave correctly. > > Reported-by: Michael Roth > Suggested-by: Stefan Hajnoczi > Cc: Michael S. Tsirkin > Cc: Cornelia Huck > Cc: Marcel Apfelbaum > Cc: Dr. David Alan Gilbert > Signed-off-by: Maxime Coquelin > --- > > I'm not sure about the property name, let me know if you have better ideas. > I didn't tested migration yet, but I wanted to share the patch while I test it. > I tested booting v2.8 and v2.7 machines with !VERSION_1 and get expected result: > - v2.8: Virtio-pci probe succeed > - v2.7: Virtio-pci probe fails > > Thanks, > Maxime > > hw/virtio/virtio-pci.c | 4 +++- > hw/virtio/virtio-pci.h | 1 + > include/hw/compat.h | 4 ++++ > 3 files changed, 8 insertions(+), 1 deletion(-) > > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c > index 521ba0b..93f6b54 100644 > --- a/hw/virtio/virtio-pci.c > +++ b/hw/virtio/virtio-pci.c > @@ -1580,7 +1580,8 @@ static void virtio_pci_device_plugged(DeviceState *d, Error **errp) > * Virtio capabilities present without > * VIRTIO_F_VERSION_1 confuses guests > */ > - if (!virtio_has_feature(vdev->host_features, VIRTIO_F_VERSION_1)) { > + if (!proxy->modern_broken && What you are de facto doing here is ignoring the features supported by the backend. Call this ->ignore_backend_features or so? > + !virtio_has_feature(vdev->host_features, VIRTIO_F_VERSION_1)) { > virtio_pci_disable_modern(proxy); > > if (!legacy) { > @@ -1852,6 +1853,7 @@ static Property virtio_pci_properties[] = { > VIRTIO_PCI_FLAG_DISABLE_PCIE_BIT, false), > DEFINE_PROP_BIT("page-per-vq", VirtIOPCIProxy, flags, > VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT, false), > + DEFINE_PROP_BOOL("x-modern-broken", VirtIOPCIProxy, modern_broken, false), What about "x-ignore-backend-features"? > DEFINE_PROP_END_OF_LIST(), > }; > > diff --git a/hw/virtio/virtio-pci.h b/hw/virtio/virtio-pci.h > index b2a996f..1dca223 100644 > --- a/hw/virtio/virtio-pci.h > +++ b/hw/virtio/virtio-pci.h > @@ -153,6 +153,7 @@ struct VirtIOPCIProxy { > int config_cap; > uint32_t flags; > bool disable_modern; > + bool modern_broken; > OnOffAuto disable_legacy; > uint32_t class_code; > uint32_t nvectors; > diff --git a/include/hw/compat.h b/include/hw/compat.h > index 0f06e11..fe11723 100644 > --- a/include/hw/compat.h > +++ b/include/hw/compat.h > @@ -18,6 +18,10 @@ > .driver = "intel-iommu",\ > .property = "x-buggy-eim",\ > .value = "true",\ > + },{\ > + .driver = "virtio-pci",\ > + .property = "x-modern-broken",\ > + .value = "on",\ This unfortunately has the same problem wrt -global propagation as the other virtio-pci compat props (unexpeced overrides). There's basically zero chance, however, that somebody will want to set this property manually (unline disable-legacy/disable-modern), and there's a patch targeting 2.8.1, so I think we can live with it. (Alternatively, you would need to set this property on all possible virtio-pci devices.) > }, > > #define HW_COMPAT_2_6 \