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

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

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

>  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[] = {