On Wed, 2012-07-04 at 18:32 +0200, Paolo Bonzini wrote: > Il 04/07/2012 18:26, Sasha Levin ha scritto: > > On Tue, 2012-07-03 at 15:19 +0200, Paolo Bonzini wrote: > >> diff --git a/include/linux/virtio_blk.h b/include/linux/virtio_blk.h > >> index e0edb40..18a1027 100644 > >> --- a/include/linux/virtio_blk.h > >> +++ b/include/linux/virtio_blk.h > >> @@ -37,8 +37,9 @@ > >> #define VIRTIO_BLK_F_RO 5 /* Disk is read-only */ > >> #define VIRTIO_BLK_F_BLK_SIZE 6 /* Block size of disk is available*/ > >> #define VIRTIO_BLK_F_SCSI 7 /* Supports scsi command passthru */ > >> -#define VIRTIO_BLK_F_FLUSH 9 /* Cache flush command support */ > >> +#define VIRTIO_BLK_F_WCE 9 /* Writeback mode enabled after reset */ > >> #define VIRTIO_BLK_F_TOPOLOGY 10 /* Topology information is available */ > >> +#define VIRTIO_BLK_F_CONFIG_WCE 11 /* Writeback mode available in config */ > > > > Wouldn't this change break any usermode code that implements virtio-blk? > > No, the change is really just clarifying the existing spec, and > mandating that virtio-blk implementations follow certain assumptions of > the Linux driver. > > In particular, the Linux driver is already assuming that the host > exposes VIRTIO_BLK_F_FLUSH if and only if it exposes a volatile write > cache. This works because if you have a writeback cache, but provide no > way to flush it, the guest driver really cannot do anything about it > anyway. Might as well treat it as writethrough, i.e. blk_queue_flush(q, 0). > > QEMU in fact has already behaved like that, and even called the flag > VIRTIO_BLK_F_WCACHE instead of VIRRTIO_BLK_F_FLUSH. There are two things going on here: 1. Rename VIRTIO_BLK_F_FLUSH to VIRTIO_BLK_F_WCE 2. Add a new VIRTIO_BLK_F_CONFIG_WCE I'm concerned that the first change is going to break compilation for any code that included linux/virtio-blk.h and used VIRTIO_BLK_F_FLUSH. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
