Re: [Qemu-devel] [PATCH] virtio-blk: set correct config size for the host driver

[Liu, Changpeng]

> -----Original Message-----
> From: Michael S. Tsirkin [mailto:address@hidden
> Sent: Tuesday, February 12, 2019 12:31 PM
> To: Liu, Changpeng <address@hidden>
> Cc: address@hidden; address@hidden; address@hidden;
> address@hidden; address@hidden
> Subject: Re: [PATCH] virtio-blk: set correct config size for the host driver
> 
> On Tue, Feb 12, 2019 at 12:25:16PM +0800, Changpeng Liu wrote:
> > Commit caa1ee43 "vhost-user-blk: add discard/write zeroes features support"
> > introduced extra fields to existing struct virtio_blk_config, when
> > migration was executed from older QEMU version to current head, it
> > will break the migration.  While here, set the correct config size
> > when initializing the host driver, for now, discard/write zeroes
> > are not supported by virtio-blk host driver, so set the config
> > size as before, users can change config size when adding the new
> > feature bits support.
> >
> > Signed-off-by: Changpeng Liu <address@hidden>
> > ---
> >  hw/block/virtio-blk.c          | 15 +++++++++++----
> >  include/hw/virtio/virtio-blk.h |  1 +
> >  2 files changed, 12 insertions(+), 4 deletions(-)
> >
> > diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> > index 9a87b3b..fac5d33 100644
> > --- a/hw/block/virtio-blk.c
> > +++ b/hw/block/virtio-blk.c
> > @@ -761,7 +761,7 @@ static void virtio_blk_update_config(VirtIODevice
> *vdev, uint8_t *config)
> >      blkcfg.alignment_offset = 0;
> >      blkcfg.wce = blk_enable_write_cache(s->blk);
> >      virtio_stw_p(vdev, &blkcfg.num_queues, s->conf.num_queues);
> > -    memcpy(config, &blkcfg, sizeof(struct virtio_blk_config));
> > +    memcpy(config, &blkcfg, s->config_size);
> >  }
> >
> >  static void virtio_blk_set_config(VirtIODevice *vdev, const uint8_t 
> > *config)
> > @@ -769,7 +769,7 @@ static void virtio_blk_set_config(VirtIODevice *vdev,
> const uint8_t *config)
> >      VirtIOBlock *s = VIRTIO_BLK(vdev);
> >      struct virtio_blk_config blkcfg;
> >
> > -    memcpy(&blkcfg, config, sizeof(blkcfg));
> > +    memcpy(&blkcfg, config, s->config_size);
> >
> >      aio_context_acquire(blk_get_aio_context(s->blk));
> >      blk_set_enable_write_cache(s->blk, blkcfg.wce != 0);
> > @@ -847,6 +847,13 @@ static void virtio_blk_set_status(VirtIODevice *vdev,
> uint8_t status)
> >      }
> >  }
> >
> > +static void virtio_blk_set_config_size(VirtIOBlock *s)
> > +{
> > +    /* VIRTIO_BLK_F_MQ is supported by host driver */
> 
> It can be disabled though.
It means the host driver can support this feature, users can use it or not.
So the config_size is same with previous old version.
> 
> It just so happens that only the addition of max_discard_seg
> crosses the next power of 2 boundary.
> 
> 
> > +    s->config_size = offsetof(struct virtio_blk_config, num_queues) +
> > +                     sizeof_field(struct virtio_blk_config, num_queues);
> > +}
> > +
> >  static void virtio_blk_save_device(VirtIODevice *vdev, QEMUFile *f)
> >  {
> >      VirtIOBlock *s = VIRTIO_BLK(vdev);
> > @@ -952,8 +959,8 @@ static void virtio_blk_device_realize(DeviceState *dev,
> Error **errp)
> >          return;
> >      }
> >
> > -    virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK,
> > -                sizeof(struct virtio_blk_config));
> > +    virtio_blk_set_config_size(s);
> > +    virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK, s->config_size);
> >
> >      s->blk = conf->conf.blk;
> >      s->rq = NULL;
> > diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
> > index 5117431..9181a93 100644
> > --- a/include/hw/virtio/virtio-blk.h
> > +++ b/include/hw/virtio/virtio-blk.h
> > @@ -51,6 +51,7 @@ typedef struct VirtIOBlock {
> >      void *rq;
> >      QEMUBH *bh;
> >      VirtIOBlkConf conf;
> > +    size_t config_size;
> >      unsigned short sector_mask;
> >      bool original_wce;
> >      VMChangeStateEntry *change;
> 
> Well assuming you are looking for a minimal change,
> go further and drop config_size completely,
> replace with a macro.
OK.
> 
> 
> > --
> > 1.9.3