Re: [Qemu-devel] [PATCH 2/2] virtio-blk: do not relay a previous driver's WCE configuration to the current

[Eric Blake <eblake@redhat.com>]

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On 09/19/2013 10:48 AM, Paolo Bonzini wrote:
> The following sequence happens:

> Hence, whenever the guest is reset, the cache mode of the disk should
> be reset to whatever was specified in the "-drive" option.  With this
> change, the Linux virtio-blk driver finds that writeback caching is
> enabled, and tells the block layer to send cache flush commands
> appropriately.
> 
> Reported-by: Rusty Russell <rusty@au1.ibm.com
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  hw/block/virtio-blk.c          | 8 ++++++--
>  include/hw/virtio/virtio-blk.h | 1 +
>  2 files changed, 7 insertions(+), 2 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

>  
>      features = vdev->guest_features;
> -    bdrv_set_enable_write_cache(s->bs, !!(features & (1 << VIRTIO_BLK_F_WCE)));
> +    if (!(features & (1 << VIRTIO_BLK_F_CONFIG_WCE))) {
> +        bdrv_set_enable_write_cache(s->bs,
> +                                    !!(features & (1 << VIRTIO_BLK_F_WCE)));
> +    }
>  }
>  
>  static void virtio_blk_save(QEMUFile *f, void *opaque)
> @@ -674,6 +678,7 @@ static int virtio_blk_device_init(VirtIODevice *vdev)
>      }
>  
>      blkconf_serial(&blk->conf, &blk->serial);
> +    s->original_wce = bdrv_enable_write_cache(blk->conf.bs);

At first, I was worried that this does 'bool = int', and whether that
was correct in all cases.  But looking further, bdrv_enable_write_cache
merely returns bs->enable_write_cache (also an int), but that all
assignments to bs->enable_write_cache are careful to only assign 0 or 1.
 A followup patch that changes the types to bool might be in order, but
doesn't invalidate this patch.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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