Re: [Qemu-devel] [PULL v3 14/15] virtio-pci: fix migration for pci bus master

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Mon, 29 Sep 2014 19:15:05 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Sep 22, 2014 at 07:28:57PM +0200, Greg Kurz wrote:
> > On Thu, 18 Sep 2014 21:54:58 +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,
> > 
> > This commit prevents pseries to boot.
> 
> OK I'll revert for now.
> 

It may be possible to change the way SLOF configures the virtq so
that it wouldn't suffer from the device being reset. We have some
time left before 2.2. I have a tentative patch for SLOF that I'll
be glad to share with Nikunj ASAP.

> > SLOF complains with the following messages:
> > 
> > Trying to load:  from: /pci@800000020000000/scsi@0 ... virtioblk_read failed! status = 255
> > virtioblk_read failed! status = 255
> > virtioblk_read failed! status = 255
> > ...
> > 
> > I'll try to debug some more.
> > 
> > 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[] = {
>