Re: [Qemu-devel] [PATCH] virtio-balloon: fix QEMU 4.0 config size migration incompatibility

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Wolfgang Bumiller (w.bumiller@proxmox.com) wrote:
> On Wed, Jul 10, 2019 at 04:14:40PM +0200, Stefan Hajnoczi wrote:
> > The virtio-balloon config size changed in QEMU 4.0 even for existing
> > machine types.  Migration from QEMU 3.1 to 4.0 can fail in some
> > circumstances with the following error:
> > 
> >   qemu-system-x86_64: get_pci_config_device: Bad config data: i=0x10 read: a1 device: 1 cmask: ff wmask: c0 w1cmask:0
> > 
> > This happens because the virtio-balloon config size affects the VIRTIO
> > Legacy I/O Memory PCI BAR size.
> > 
> > Introduce a qdev property called "qemu-4-0-config-size" and enable it
> > only for the QEMU 4.0 machine types.  This way <4.0 machine types use
> > the old size, 4.0 uses the larger size, and >4.0 machine types use the
> > appropriate size depending on enabled virtio-balloon features.
> > 
> > Live migration to and from old QEMUs to QEMU 4.1 works again as long as
> > a versioned machine type is specified (do not use just "pc"!).
> > 
> > Originally-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> 
> Tested-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
> 
> Works with my otherwise failing VM from 3.0.1 -> patched-4.0.
> Somehow I missed the `DEFINE_PROP_*` macros, sorry, and thanks for
> fixing this up.

Thanks again for spotting it!

Dave

> > ---
> >  include/hw/virtio/virtio-balloon.h |  2 ++
> >  hw/core/machine.c                  |  2 ++
> >  hw/virtio/virtio-balloon.c         | 28 +++++++++++++++++++++++++---
> >  3 files changed, 29 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/hw/virtio/virtio-balloon.h b/include/hw/virtio/virtio-balloon.h
> > index 1afafb12f6..5a99293a45 100644
> > --- a/include/hw/virtio/virtio-balloon.h
> > +++ b/include/hw/virtio/virtio-balloon.h
> > @@ -71,6 +71,8 @@ typedef struct VirtIOBalloon {
> >      int64_t stats_poll_interval;
> >      uint32_t host_features;
> >      PartiallyBalloonedPage *pbp;
> > +
> > +    bool qemu_4_0_config_size;
> >  } VirtIOBalloon;
> >  
> >  #endif
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index 2be19ec0cd..c4ead16010 100644
> > --- a/hw/core/machine.c
> > +++ b/hw/core/machine.c
> > @@ -34,6 +34,7 @@ GlobalProperty hw_compat_4_0[] = {
> >      { "virtio-vga",     "edid", "false" },
> >      { "virtio-gpu-pci", "edid", "false" },
> >      { "virtio-device", "use-started", "false" },
> > +    { "virtio-balloon-device", "qemu-4-0-config-size", "true" },
> >  };
> >  const size_t hw_compat_4_0_len = G_N_ELEMENTS(hw_compat_4_0);
> >  
> > @@ -49,6 +50,7 @@ GlobalProperty hw_compat_3_1[] = {
> >      { "usb-tablet", "serial", "42" },
> >      { "virtio-blk-device", "discard", "false" },
> >      { "virtio-blk-device", "write-zeroes", "false" },
> > +    { "virtio-balloon-device", "qemu-4-0-config-size", "false" },
> >  };
> >  const size_t hw_compat_3_1_len = G_N_ELEMENTS(hw_compat_3_1);
> >  
> > diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> > index 11fad86d64..e85d1c0d5c 100644
> > --- a/hw/virtio/virtio-balloon.c
> > +++ b/hw/virtio/virtio-balloon.c
> > @@ -615,6 +615,22 @@ virtio_balloon_free_page_report_notify(NotifierWithReturn *n, void *data)
> >      return 0;
> >  }
> >  
> > +static size_t virtio_balloon_config_size(VirtIOBalloon *s)
> > +{
> > +    uint64_t features = s->host_features;
> > +
> > +    if (s->qemu_4_0_config_size) {
> > +        return sizeof(struct virtio_balloon_config);
> > +    }
> > +    if (virtio_has_feature(features, VIRTIO_BALLOON_F_PAGE_POISON)) {
> > +        return sizeof(struct virtio_balloon_config);
> > +    }
> > +    if (virtio_has_feature(features, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> > +        return offsetof(struct virtio_balloon_config, poison_val);
> > +    }
> > +    return offsetof(struct virtio_balloon_config, free_page_report_cmd_id);
> > +}
> > +
> >  static void virtio_balloon_get_config(VirtIODevice *vdev, uint8_t *config_data)
> >  {
> >      VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
> > @@ -635,7 +651,7 @@ static void virtio_balloon_get_config(VirtIODevice *vdev, uint8_t *config_data)
> >      }
> >  
> >      trace_virtio_balloon_get_config(config.num_pages, config.actual);
> > -    memcpy(config_data, &config, sizeof(struct virtio_balloon_config));
> > +    memcpy(config_data, &config, virtio_balloon_config_size(dev));
> >  }
> >  
> >  static int build_dimm_list(Object *obj, void *opaque)
> > @@ -679,7 +695,7 @@ static void virtio_balloon_set_config(VirtIODevice *vdev,
> >      uint32_t oldactual = dev->actual;
> >      ram_addr_t vm_ram_size = get_current_ram_size();
> >  
> > -    memcpy(&config, config_data, sizeof(struct virtio_balloon_config));
> > +    memcpy(&config, config_data, virtio_balloon_config_size(dev));
> >      dev->actual = le32_to_cpu(config.actual);
> >      if (dev->actual != oldactual) {
> >          qapi_event_send_balloon_change(vm_ram_size -
> > @@ -766,7 +782,7 @@ static void virtio_balloon_device_realize(DeviceState *dev, Error **errp)
> >      int ret;
> >  
> >      virtio_init(vdev, "virtio-balloon", VIRTIO_ID_BALLOON,
> > -                sizeof(struct virtio_balloon_config));
> > +                virtio_balloon_config_size(s));
> >  
> >      ret = qemu_add_balloon_handler(virtio_balloon_to_target,
> >                                     virtio_balloon_stat, s);
> > @@ -897,6 +913,12 @@ static Property virtio_balloon_properties[] = {
> >                      VIRTIO_BALLOON_F_DEFLATE_ON_OOM, false),
> >      DEFINE_PROP_BIT("free-page-hint", VirtIOBalloon, host_features,
> >                      VIRTIO_BALLOON_F_FREE_PAGE_HINT, false),
> > +    /* QEMU 4.0 accidentally changed the config size even when free-page-hint
> > +     * is disabled, resulting in QEMU 3.1 migration incompatibility.  This
> > +     * property retains this quirk for QEMU 4.1 machine types.
> > +     */
> > +    DEFINE_PROP_BOOL("qemu-4-0-config-size", VirtIOBalloon,
> > +                     qemu_4_0_config_size, false),
> >      DEFINE_PROP_LINK("iothread", VirtIOBalloon, iothread, TYPE_IOTHREAD,
> >                       IOThread *),
> >      DEFINE_PROP_END_OF_LIST(),
> > -- 
> > 2.21.0
> > 
> > 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK