1.1 --- a/xen/include/public/io/blkif.h	Thu Nov 09 16:49:59 2006 +0000
     1.2 +++ b/xen/include/public/io/blkif.h	Thu Nov 09 16:50:44 2006 +0000
     1.3 @@ -47,8 +47,22 @@
     1.4  #endif
     1.5  #define blkif_sector_t uint64_t
     1.6  
     1.7 -#define BLKIF_OP_READ      0
     1.8 -#define BLKIF_OP_WRITE     1
     1.9 +/*
    1.10 + * REQUEST CODES.
    1.11 + */
    1.12 +#define BLKIF_OP_READ              0
    1.13 +#define BLKIF_OP_WRITE             1
    1.14 +/*
    1.15 + * Recognised only if "feature-barrier" is present in backend xenbus info.
    1.16 + * The "feature_barrier" node contains a boolean indicating whether barrier
    1.17 + * requests are likely to succeed or fail. Either way, a barrier request
    1.18 + * may fail at any time with BLKIF_RSP_EOPNOTSUPP if it is unsupported by
    1.19 + * the underlying block-device hardware. The boolean simply indicates whether
    1.20 + * or not it is worthwhile for the frontend to attempt barrier requests.
    1.21 + * If a backend does not recognise BLKIF_OP_WRITE_BARRIER, it should *not*
    1.22 + * create the "feature-barrier" node!
    1.23 + */
    1.24 +#define BLKIF_OP_WRITE_BARRIER     2
    1.25  
    1.26  /*
    1.27   * Maximum scatter/gather segments per request.
    1.28 @@ -79,8 +93,15 @@ struct blkif_response {
    1.29  };
    1.30  typedef struct blkif_response blkif_response_t;
    1.31  
    1.32 -#define BLKIF_RSP_ERROR  -1 /* non-specific 'error' */
    1.33 -#define BLKIF_RSP_OKAY    0 /* non-specific 'okay'  */
    1.34 +/*
    1.35 + * STATUS RETURN CODES.
    1.36 + */
    1.37 + /* Operation not supported (only happens on barrier writes). */
    1.38 +#define BLKIF_RSP_EOPNOTSUPP  -2
    1.39 + /* Operation failed for some unspecified reason (-EIO). */
    1.40 +#define BLKIF_RSP_ERROR       -1
    1.41 + /* Operation completed successfully. */
    1.42 +#define BLKIF_RSP_OKAY         0
    1.43  
    1.44  /*
    1.45   * Generate blkif ring structures and types.