RE: [Xen-devel] disk io request structures

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> Hi,
> Following is the structure of an IO request which is inserted in the
shared IO
> ring.
> 
> include/xen/interface/io/blkif.h
> 
> struct blkif_request {
>     uint8_t        operation;    /* BLKIF_OP_???
*/
>     uint8_t        nr_segments;  /* number of segments
*/
>     blkif_vdev_t   handle;       /* only for read/write requests
*/
>     uint64_t       id;           /* private guest value, echoed in
resp  */
>     blkif_sector_t sector_number;/* start sector idx on disk (r/w
only)  */
>     struct blkif_request_segment {
>         grant_ref_t gref;        /* reference to I/O buffer frame
*/
>         /* @first_sect: first sector in frame to transfer (inclusive).
*/
>         /* @last_sect: last sector in frame to transfer (inclusive).
*/
>         uint8_t     first_sect, last_sect;
>     } seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> };
> 
> Sorry, if my questions seem obvious or trivial. I am a newbie in xen
> development. Right now, me and my group are trying to understand the
disk io
> mechanism in XEN. We intend to modify the existing system. We expect
better
> performance after this modification.
> 
> So, in the above structure,
>  1. what is a "segment" referred in the structure.
>  2. Why is there an array of blkif_request_segment (why not a single
> instance). What is it's purpose?
>  3. In the structure blkif_request_segment, are the first_sect and
last_sect
> physical sector numbers
> 

Each entry on the ring contains the details of the operation. Each
operation is going to be either a read from memory or a write to memory.
The ring entry contains a list of memory pages that are granted to Dom0.

The first_sect and last_sect are worked out by thinking of each page as
holding 8 sectors (page size is 4096 bytes, sectors are 512 bytes).
first_sect and last_sect refer to the sector number offset inside that
frame, so if the address of your buffer was offset 0 then first_sect
would be 0, if it was offset 512 then first_sect would be 1, etc. Your
buffer obviously has to be aligned to a 512 byte boundary (which is a
pain for Windows!).

Probably the best way to describe it is an example. Suppose you want to
read sectors 123-134 (11 sectors) into virtual memory starting at
address 0x12345600 (length 11 * 512 = 5632). You would do the following:

1. Get the pfn's of the memory. Eg virtual address 0x12345600-0x12346BFF
might map to pages 0x4444 and 0x5555
2. Get a grant ref and grant access to those pfn's. Your grant refs
might be 123 and 456
3. Set the members of the struct as follow:
   . operation = read
   . nr_segments = 2 (2 pages)
   . sector_number = 123
   . handle = I forget what this is for...
   . id = your own id for you to track this request when you get the
response back
   . seg[0].gref = 123
   . seg[0].first_sect = 3 (offset 0x600 in frame)
   . seg[0].last_sect = 7
   . seg[1].gref = 456
   . seg[1].first_sect = 0
   . seg[1].last_sect = 5

Please excuse any arithmetic errors I might have made above, but the
logic should be right.

James

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