Re: [Qemu-devel] [PATCH] virtio-pci: fix migration for pci bus master

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 06, 2014 at 06:46:17PM +0200, Greg Kurz wrote:
> On Mon, 6 Oct 2014 19:26:21 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > On Mon, Oct 06, 2014 at 04:51:35PM +0200, Greg Kurz wrote:
> > > On Wed, 17 Sep 2014 20:39:25 +0300
> > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > 
> > > > On Wed, Sep 17, 2014 at 07:21:09PM +0200, Greg Kurz wrote:
> > > > > On Sun, 14 Sep 2014 21:30:36 +0300
> > > > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > > > 
> > > > > > Current support for bus master (clearing OK bit)
> > > > > > together with the need to support guests which do not
> > > > > > enable PCI bus mastering, leads to extra state in
> > > > > > VIRTIO_PCI_FLAG_BUS_MASTER_BUG bit, which isn't robust
> > > > > > in case of cross-version migration for the case when
> > > > > > guests use the device before setting DRIVER_OK.
> > > > > > 
> > > > > > Rip out VIRTIO_PCI_FLAG_BUS_MASTER_BUG and implement a simpler
> > > > > > work-around: treat clearing of PCI_COMMAND as a virtio reset.  Old
> > > > > > guests never touch this bit so they will work.
> > > > > > 
> > > > > > As reset clears device status, DRIVER and MASTER bits are
> > > > > > now in sync, so we can fix up cross-version migration simply
> > > > > > by synchronising them, without need to detect a buggy guest
> > > > > > explicitly.
> > > > > > 
> > > > > > Drop tracking VIRTIO_PCI_FLAG_BUS_MASTER_BUG completely.
> > > > > > 
> > > > > > As reset makes the device quiescent, in the future we'll be able to drop
> > > > > > checking OK bit in a bunch of places.
> > > > > > 
> > > > > > Cc: Jason Wang <jasowang@redhat.com>
> > > > > > Cc: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > > > ---
> > > > > 
> > > > > Hi Michael,
> > > > > 
> > > > > I am not quite sure how to test this patch with my pseries based setup...
> > > > > Migrating from qemu-2.1 to qemu-master ?
> > > > > 
> > > > > Cheers.
> > > > > 
> > > > > --
> > > > > Greg
> > > > 
> > > > Exactly. And back! Pls don't forget to specify the 2.1 machine type.
> > > > Thanks!
> > > > 
> > > 
> > > Michael,
> > > 
> > > Nikunj and I had started to investigate the pseries breakage: the QEMU
> > > originated reset brought by this patch clears the vq and breaks SLOF.
> > > This isn't a surprise since reset should always come from the driver,
> > > not the device.
> > > 
> > > Since commit 45363e46aeebfc99753389649eac7c7fc22bfe52 has reverted this
> > > patch, QEMU works again for pseries and virtio. :)
> > > 
> > > So back to the initial issue, I've tried to migrate a pseries-2.1 guest running
> > > rhel65, from QEMU v2.1.2 to QEMU master, back and forth, several times and it
> > > always succeeded... what symptom this patch was expected to fix ?
> > > 
> > > Cheers.
> > > 
> > > --
> > > Greg
> > > 
> > > > > >  hw/virtio/virtio-pci.c | 39 ++++++++++++++++++++-------------------
> > > > > >  1 file changed, 20 insertions(+), 19 deletions(-)
> > > > > > 
> > > > > > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > > > > > index a827cd4..f560814 100644
> > > > > > --- a/hw/virtio/virtio-pci.c
> > > > > > +++ b/hw/virtio/virtio-pci.c
> > > > > > @@ -86,9 +86,6 @@
> > > > > >   * 12 is historical, and due to x86 page size. */
> > > > > >  #define VIRTIO_PCI_QUEUE_ADDR_SHIFT    12
> > > > > > 
> > > > > > -/* Flags track per-device state like workarounds for quirks in older guests. */
> > > > > > -#define VIRTIO_PCI_FLAG_BUS_MASTER_BUG  (1 << 0)
> > > > > > -
> > > > > >  static void virtio_pci_bus_new(VirtioBusState *bus, size_t bus_size,
> > > > > >                                 VirtIOPCIProxy *dev);
> > > > > > 
> > > > > > @@ -323,14 +320,6 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
> > > > > >                                       proxy->pci_dev.config[PCI_COMMAND] |
> > > > > >                                       PCI_COMMAND_MASTER, 1);
> > > > > >          }
> > > > > > -
> > > > > > -        /* Linux before 2.6.34 sets the device as OK without enabling
> > > > > > -           the PCI device bus master bit. In this case we need to disable
> > > > > > -           some safety checks. */
> > > > > > -        if ((val & VIRTIO_CONFIG_S_DRIVER_OK) &&
> > > > > > -            !(proxy->pci_dev.config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
> > > > > > -            proxy->flags |= VIRTIO_PCI_FLAG_BUS_MASTER_BUG;
> > > > > > -        }
> > > > > >          break;
> > > > > >      case VIRTIO_MSI_CONFIG_VECTOR:
> > > > > >          msix_vector_unuse(&proxy->pci_dev, vdev->config_vector);
> > > > > > @@ -480,13 +469,18 @@ static void virtio_write_config(PCIDevice *pci_dev, uint32_t address,
> > > > > >      VirtIOPCIProxy *proxy = DO_UPCAST(VirtIOPCIProxy, pci_dev, pci_dev);
> > > > > >      VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
> > > > > > 
> > > > > > +    uint8_t cmd = proxy->pci_dev.config[PCI_COMMAND];
> > > > > > +
> > > > > >      pci_default_write_config(pci_dev, address, val, len);
> > > > > > 
> > > > > >      if (range_covers_byte(address, len, PCI_COMMAND) &&
> > > > > >          !(pci_dev->config[PCI_COMMAND] & PCI_COMMAND_MASTER) &&
> > > > > > -        !(proxy->flags & VIRTIO_PCI_FLAG_BUS_MASTER_BUG)) {
> > > > > > +        (cmd & PCI_COMMAND_MASTER)) {
> > > > > > +        /* Bus driver disables bus mastering - make it act
> > > > > > +         * as a kind of reset to render the device quiescent. */
> > > > > >          virtio_pci_stop_ioeventfd(proxy);
> > > > > > -        virtio_set_status(vdev, vdev->status & ~VIRTIO_CONFIG_S_DRIVER_OK);
> > > > > > +        virtio_reset(vdev);
> > > > > > +        msix_unuse_all_vectors(&proxy->pci_dev);
> > > > > >      }
> > > > > >  }
> > > > > > 
> > > > > > @@ -895,11 +889,19 @@ static void virtio_pci_vmstate_change(DeviceState *d, bool running)
> > > > > >      VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
> > > > > > 
> > > > > >      if (running) {
> > > > > > -        /* Try to find out if the guest has bus master disabled, but is
> > > > > > -           in ready state. Then we have a buggy guest OS. */
> > > > > > -        if ((vdev->status & VIRTIO_CONFIG_S_DRIVER_OK) &&
> > > > > > -            !(proxy->pci_dev.config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
> > > > > > -            proxy->flags |= VIRTIO_PCI_FLAG_BUS_MASTER_BUG;
> > > > > > +        /* Linux before 2.6.34 drives the device without enabling
> > > > > > +           the PCI device bus master bit. Enable it automatically
> > > > > > +           for the guest. This is a PCI spec violation but so is
> > > > > > +           initiating DMA with bus master bit clear.
> > > > > > +           Note: this only makes a difference when migrating
> > > > > > +           across QEMU versions from an old QEMU, as for new QEMU
> > > > > > +           bus master and driver bits are always in sync.
> > > > > > +           TODO: consider enabling conditionally for compat machine types. */
> > > > > > +        if (vdev->status & (VIRTIO_CONFIG_S_ACKNOWLEDGE |
> > > > > > +                            VIRTIO_CONFIG_S_DRIVER)) {
> > > > > > +            pci_default_write_config(&proxy->pci_dev, PCI_COMMAND,
> > > > > > +                                     proxy->pci_dev.config[PCI_COMMAND] |
> > > > > > +                                     PCI_COMMAND_MASTER, 1);
> > > > > >          }
> > > > > >          virtio_pci_start_ioeventfd(proxy);
> > > > > >      } else {
> > > > > > @@ -1040,7 +1042,6 @@ static void virtio_pci_reset(DeviceState *qdev)
> > > > > >      virtio_pci_stop_ioeventfd(proxy);
> > > > > >      virtio_bus_reset(bus);
> > > > > >      msix_unuse_all_vectors(&proxy->pci_dev);
> > > > > > -    proxy->flags &= ~VIRTIO_PCI_FLAG_BUS_MASTER_BUG;
> > > > > >  }
> > > > > > 
> > > > > >  static Property virtio_pci_properties[] = {
> > > > 
> > 
> > 
> > It's a race condition: you have to catch it migrating
> > in the window before DRIVER_OK is set.
> > 
> 
> Ok I see... when DRIVER and MASTER bits are not in sync. I'll patch QEMU to
> trigger a migration in this window then.
> 
> Thanks.

BTW I reverted that patch, and to fix migration, I'm thinking
about applying the following patch on top of master.

Would appreciate testing cross-version migration (2.1 to master)
with this patch applied.



diff --git a/hw/virtio/virtio-pci.h b/hw/virtio/virtio-pci.h
index 1cea157..8873b6d 100644
--- a/hw/virtio/virtio-pci.h
+++ b/hw/virtio/virtio-pci.h
@@ -53,6 +53,11 @@ typedef struct VirtioBusClass VirtioPCIBusClass;
 #define VIRTIO_PCI_BUS_CLASS(klass) \
         OBJECT_CLASS_CHECK(VirtioPCIBusClass, klass, TYPE_VIRTIO_PCI_BUS)
 
+/* Need to activate work-arounds for buggy guests at vmstate load. */
+#define VIRTIO_PCI_FLAG_BUS_MASTER_BUG_MIGRATION_BIT  0
+#define VIRTIO_PCI_FLAG_BUS_MASTER_BUG_MIGRATION \
+    (1 << VIRTIO_PCI_FLAG_BUS_MASTER_BUG_MIGRATION_BIT)
+
 /* Performance improves when virtqueue kick processing is decoupled from the
  * vcpu thread using ioeventfd for some devices. */
 #define VIRTIO_PCI_FLAG_USE_IOEVENTFD_BIT 1
diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index bae023a..e07b6c4 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -312,6 +312,11 @@ bool e820_get_entry(int, uint32_t, uint64_t *, uint64_t *);
             .driver   = "intel-hda",\
             .property = "old_msi_addr",\
             .value    = "on",\
+        },\
+        {\
+            .driver   = "virtio-pci",\
+            .property = "virtio-pci-bus-master-bug-migration",\
+            .value    = "on",\
         }
 
 #define PC_COMPAT_2_0 \
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index a827cd4..a499a3c 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -86,9 +86,6 @@
  * 12 is historical, and due to x86 page size. */
 #define VIRTIO_PCI_QUEUE_ADDR_SHIFT    12
 
-/* Flags track per-device state like workarounds for quirks in older guests. */
-#define VIRTIO_PCI_FLAG_BUS_MASTER_BUG  (1 << 0)
-
 static void virtio_pci_bus_new(VirtioBusState *bus, size_t bus_size,
                                VirtIOPCIProxy *dev);
 
@@ -323,14 +320,6 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
                                      proxy->pci_dev.config[PCI_COMMAND] |
                                      PCI_COMMAND_MASTER, 1);
         }
-
-        /* Linux before 2.6.34 sets the device as OK without enabling
-           the PCI device bus master bit. In this case we need to disable
-           some safety checks. */
-        if ((val & VIRTIO_CONFIG_S_DRIVER_OK) &&
-            !(proxy->pci_dev.config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
-            proxy->flags |= VIRTIO_PCI_FLAG_BUS_MASTER_BUG;
-        }
         break;
     case VIRTIO_MSI_CONFIG_VECTOR:
         msix_vector_unuse(&proxy->pci_dev, vdev->config_vector);
@@ -483,8 +472,7 @@ static void virtio_write_config(PCIDevice *pci_dev, uint32_t address,
     pci_default_write_config(pci_dev, address, val, len);
 
     if (range_covers_byte(address, len, PCI_COMMAND) &&
-        !(pci_dev->config[PCI_COMMAND] & PCI_COMMAND_MASTER) &&
-        !(proxy->flags & VIRTIO_PCI_FLAG_BUS_MASTER_BUG)) {
+        !(pci_dev->config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
         virtio_pci_stop_ioeventfd(proxy);
         virtio_set_status(vdev, vdev->status & ~VIRTIO_CONFIG_S_DRIVER_OK);
     }
@@ -895,11 +883,15 @@ static void virtio_pci_vmstate_change(DeviceState *d, bool running)
     VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
 
     if (running) {
-        /* Try to find out if the guest has bus master disabled, but is
-           in ready state. Then we have a buggy guest OS. */
-        if ((vdev->status & VIRTIO_CONFIG_S_DRIVER_OK) &&
+        /* Old QEMU versions did not set bus master enable on status write.
+         * Detect DRIVER set and enable it.
+         */
+        if ((proxy->flags & VIRTIO_PCI_FLAG_BUS_MASTER_BUG_MIGRATION) &&
+            (vdev->status & VIRTIO_CONFIG_S_DRIVER) &&
             !(proxy->pci_dev.config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
-            proxy->flags |= VIRTIO_PCI_FLAG_BUS_MASTER_BUG;
+            pci_default_write_config(&proxy->pci_dev, PCI_COMMAND,
+                                     proxy->pci_dev.config[PCI_COMMAND] |
+                                     PCI_COMMAND_MASTER, 1);
         }
         virtio_pci_start_ioeventfd(proxy);
     } else {
@@ -1040,10 +1032,11 @@ static void virtio_pci_reset(DeviceState *qdev)
     virtio_pci_stop_ioeventfd(proxy);
     virtio_bus_reset(bus);
     msix_unuse_all_vectors(&proxy->pci_dev);
-    proxy->flags &= ~VIRTIO_PCI_FLAG_BUS_MASTER_BUG;
 }
 
 static Property virtio_pci_properties[] = {
+    DEFINE_PROP_BIT("virtio-pci-bus-master-bug-migration", VirtIOPCIProxy, flags,
+                    VIRTIO_PCI_FLAG_BUS_MASTER_BUG_MIGRATION_BIT, false),
     DEFINE_VIRTIO_COMMON_FEATURES(VirtIOPCIProxy, host_features),
     DEFINE_PROP_END_OF_LIST(),
 };