Re: [Qemu-devel] [PATCH] virtio/vhost: reset dev->log after syncing

["Marc-André Lureau" <marcandre.lureau@redhat.com>]

Hi

----- Original Message -----
> vhost_log_put() is called to decomission the dirty log between qemu and
> a vhost device when stopping the device. Such a call can happen from
> migration_completion().
> 
> Present code sets dev->log_size to zero too early in vhost_log_put(),
> causing the sync check to always return false. As a consequence, the
> last pass on the dirty bitmap never happens at the end of migration.
> 
> If a vhost device was busy (writing to guest memory) until the last
> moments before vhost_virtqueue_stop(), this error will result in guest
> memory corruption (at least) following migrations.
> 
> Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
> ---
>  hw/virtio/vhost.c |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 5fd69f0..ddc42f0 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -375,8 +375,6 @@ static void vhost_log_put(struct vhost_dev *dev, bool
> sync)
>      if (!log) {
>          return;
>      }
> -    dev->log = NULL;
> -    dev->log_size = 0;
>  


Good catch. This reminds me of another patch, but I can't find it.

What if we replace dev->log_size with log->size below? 

(and I don't see a clear reason why dev->log_size would be different from "log ? log->size : 0", am I missing something?)

>      --log->refcnt;
>      if (log->refcnt == 0) {
> @@ -396,6 +394,9 @@ static void vhost_log_put(struct vhost_dev *dev, bool
> sync)
>  
>          g_free(log);
>      }
> +
> +    dev->log = NULL;
> +    dev->log_size = 0;

>  }
>  
>  static bool vhost_dev_log_is_shared(struct vhost_dev *dev)
> --
> 1.7.1
> 
>