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[Qemu-devel] [PATCH v2] virtio-pci: Fix cross-version migration with old


From: Maxime Coquelin
Subject: [Qemu-devel] [PATCH v2] virtio-pci: Fix cross-version migration with older machines
Date: Wed, 14 Dec 2016 17:19:58 +0100

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
QEMU version with virtio-pci modern capabilities enabled to a v2.8
QEMU version.

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 <address@hidden>
Suggested-by: Stefan Hajnoczi <address@hidden>
Cc: Michael S. Tsirkin <address@hidden>
Cc: Cornelia Huck <address@hidden>
Cc: Dr. David Alan Gilbert <address@hidden>
Reviewed-by: Marcel Apfelbaum <address@hidden>
Reviewed-by: Stefan Hajnoczi <address@hidden>
Tested-by: Michael Roth <address@hidden>
Signed-off-by: Maxime Coquelin <address@hidden>
---

Thanks for the reviews and tests.
This version changes the naming as proposed by Michael T.
and Cornelia, and fixies commit message.

 hw/virtio/virtio-pci.c | 5 ++++-
 hw/virtio/virtio-pci.h | 1 +
 include/hw/compat.h    | 4 ++++
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 521ba0b..21c2b9d 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->ignore_backend_features &&
+            !virtio_has_feature(vdev->host_features, VIRTIO_F_VERSION_1)) {
         virtio_pci_disable_modern(proxy);
 
         if (!legacy) {
@@ -1852,6 +1853,8 @@ 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-ignore-backend-features", VirtIOPCIProxy,
+                     ignore_backend_features, false),
     DEFINE_PROP_END_OF_LIST(),
 };
 
diff --git a/hw/virtio/virtio-pci.h b/hw/virtio/virtio-pci.h
index b2a996f..5e07886 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 ignore_backend_features;
     OnOffAuto disable_legacy;
     uint32_t class_code;
     uint32_t nvectors;
diff --git a/include/hw/compat.h b/include/hw/compat.h
index 0f06e11..8dfc7a3 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-ignore-backend-features",\
+        .value    = "on",\
     },
 
 #define HW_COMPAT_2_6 \
-- 
2.9.3




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