Re: [PATCH V2 1/3] xen-blkfront: add BLKIF_OP_TRIM and backend type flags

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 08/18/2011 02:34 AM, Li Dongyang wrote:
> This adds the BLKIF_OP_TRIM for blkfront and blkback, also 2 flags telling
> us the type of the backend, used in blkback to determine what to do when we
> see a trim request.
> Part of the patch is just taken from Owen Smith, Thanks
>
> Signed-off-by: Owen Smith <owen.smith@citrix.com>
> Signed-off-by: Li Dongyang <lidongyang@novell.com>
> ---
>  include/xen/interface/io/blkif.h |   21 +++++++++++++++++++++
>  1 files changed, 21 insertions(+), 0 deletions(-)
>
> diff --git a/include/xen/interface/io/blkif.h b/include/xen/interface/io/blkif.h
> index 3d5d6db..b92cf23 100644
> --- a/include/xen/interface/io/blkif.h
> +++ b/include/xen/interface/io/blkif.h
> @@ -57,6 +57,19 @@ typedef uint64_t blkif_sector_t;
>   * "feature-flush-cache" node!
>   */
>  #define BLKIF_OP_FLUSH_DISKCACHE   3
> +
> +/*
> + * Recognised only if "feature-trim" is present in backend xenbus info.
> + * The "feature-trim" node contains a boolean indicating whether barrier
> + * requests are likely to succeed or fail. Either way, a trim request

Barrier requests?

> + * may fail at any time with BLKIF_RSP_EOPNOTSUPP if it is unsupported by
> + * the underlying block-device hardware. The boolean simply indicates whether
> + * or not it is worthwhile for the frontend to attempt trim requests.
> + * If a backend does not recognise BLKIF_OP_TRIM, it should *not*
> + * create the "feature-trim" node!

Is all this necessary?  What happens if guests just send OP_TRIM
requests, and if the host doesn't understand them then it will fails
them with EOPNOTSUPP?  Is a TRIM request ever anything more than a hint
to the backend that certain blocks are no longer needed?

> + */
> +#define BLKIF_OP_TRIM            5
> +
>  /*
>   * Maximum scatter/gather segments per request.
>   * This is carefully chosen so that sizeof(struct blkif_ring) <= PAGE_SIZE.
> @@ -74,6 +87,11 @@ struct blkif_request_rw {
>  	} seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
>  };
>  
> +struct blkif_request_trim {
> +	blkif_sector_t sector_number;
> +	uint64_t nr_sectors;
> +};
> +
>  struct blkif_request {
>  	uint8_t        operation;    /* BLKIF_OP_???                         */
>  	uint8_t        nr_segments;  /* number of segments                   */
> @@ -81,6 +99,7 @@ struct blkif_request {
>  	uint64_t       id;           /* private guest value, echoed in resp  */
>  	union {
>  		struct blkif_request_rw rw;
> +		struct blkif_request_trim trim;
>  	} u;
>  };
>  
> @@ -109,6 +128,8 @@ DEFINE_RING_TYPES(blkif, struct blkif_request, struct blkif_response);
>  #define VDISK_CDROM        0x1
>  #define VDISK_REMOVABLE    0x2
>  #define VDISK_READONLY     0x4
> +#define VDISK_FILE_BACKEND 0x8
> +#define VDISK_PHY_BACKEND  0x10

What are these for?  Why does a frontend care about these?

    J