[Xen-devel] Ping: [PATCH] vscsiif: allow larger segments-per-request values

["Jan Beulich" <JBeulich@xxxxxxxx>]

Does silence here imply agreement?

Jan

>>> On 27.11.12 at 12:37, Jan Beulich wrote:
> At least certain tape devices require fixed size blocks to be operated
> upon, i.e. breaking up of I/O requests is not permitted. Consequently
> we need an interface extension that (leaving aside implementation
> limitations) doesn't impose a limit on the number of segments that can
> be associated with an individual request.
> 
> This, in turn, excludes the blkif extension FreeBSD folks implemented,
> as that still imposes an upper limit (the actual I/O request still
> specifies the full number of segments - as an 8-bit quantity -, and
> subsequent ring slots get used to carry the excess segment
> descriptors).
> 
> The alternative therefore is to allow the frontend to pre-set segment
> descriptors _before_ actually issuing the I/O request. I/O will then
> be done by the backend for the accumulated set of segments.
> 
> To properly associate segment preset operations with the main request,
> the rqid-s between them should match (originally I had hoped to use
> this to avoid producing individual responses for the pre-set
> operations, but that turned out to violate the underlying shared ring
> implementation).
> 
> Negotiation of the maximum number of segments a particular backend
> implementation supports happens through a new "segs-per-req" xenstore
> node.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> As I have no plans to backport this to the 2.6.18 tree, I'm attaching
> for reference the full kernel side patch we're intending to use.
> 
> --- a/xen/include/public/io/vscsiif.h
> +++ b/xen/include/public/io/vscsiif.h
> @@ -34,6 +34,7 @@
>  #define VSCSIIF_ACT_SCSI_CDB         1    /* SCSI CDB command */
>  #define VSCSIIF_ACT_SCSI_ABORT       2    /* SCSI Device(Lun) Abort*/
>  #define VSCSIIF_ACT_SCSI_RESET       3    /* SCSI Device(Lun) Reset*/
> +#define VSCSIIF_ACT_SCSI_SG_PRESET   4    /* Preset SG elements */
>  
>  /*
>   * Maximum scatter/gather segments per request.
> @@ -50,6 +51,12 @@
>  #define VSCSIIF_MAX_COMMAND_SIZE         16
>  #define VSCSIIF_SENSE_BUFFERSIZE         96
>  
> +struct scsiif_request_segment {
> +    grant_ref_t gref;
> +    uint16_t offset;
> +    uint16_t length;
> +};
> +typedef struct scsiif_request_segment vscsiif_segment_t;
>  
>  struct vscsiif_request {
>      uint16_t rqid;          /* private guest value, echoed in resp  */
> @@ -66,18 +73,26 @@ struct vscsiif_request {
>                                           DMA_NONE(3) requests  */
>      uint8_t nr_segments;              /* Number of pieces of scatter-gather 
> */
>  
> -    struct scsiif_request_segment {
> -        grant_ref_t gref;
> -        uint16_t offset;
> -        uint16_t length;
> -    } seg[VSCSIIF_SG_TABLESIZE];
> +    vscsiif_segment_t seg[VSCSIIF_SG_TABLESIZE];
>      uint32_t reserved[3];
>  };
>  typedef struct vscsiif_request vscsiif_request_t;
>  
> +#define VSCSIIF_SG_LIST_SIZE ((sizeof(vscsiif_request_t) - 4) \
> +                              / sizeof(vscsiif_segment_t))
> +
> +struct vscsiif_sg_list {
> +    /* First two fields must match struct vscsiif_request! */
> +    uint16_t rqid;          /* private guest value, must match main req */
> +    uint8_t act;            /* VSCSIIF_ACT_SCSI_SG_PRESET */
> +    uint8_t nr_segments;    /* Number of pieces of scatter-gather */
> +    vscsiif_segment_t seg[VSCSIIF_SG_LIST_SIZE];
> +};
> +typedef struct vscsiif_sg_list vscsiif_sg_list_t;
> +
>  struct vscsiif_response {
>      uint16_t rqid;
> -    uint8_t padding;
> +    uint8_t act;               /* valid only when backend supports 
> SG_PRESET */
>      uint8_t sense_len;
>      uint8_t sense_buffer[VSCSIIF_SENSE_BUFFERSIZE];
>      int32_t rslt;
> 
> 
> 




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