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

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

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!

> >  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[] = {