Re: [Qemu-devel] [RFC] introduce a dynamic library to expose qemu block API

[Blue Swirl]

On Wed, Jul 18, 2012 at 8:51 AM, Wenchao Xia <address@hidden> wrote:
>   Hi, following is API draft, prototypes were taken from qemu/block.h,
> and the API prefix is changed frpm bdrv to qbdrvs, to declare related
> object is BlockDriverState, not BlockDriver. One issue here is it may
> require include block_int.h, which is not LGPL2 licensed yet.
>   API format is kept mostly the same with qemu generic block layer, to
> make it easier for implement, and easier to make qemu migrate on it if
> possible.
>
>
> /* structure init and uninit */
> BlockDriverState *qbdrvs_new(const char *device_name);
> void qbdrvs_delete(BlockDriverState *bs);
>
>
> /* file open and close */
> int qbdrvs_open(BlockDriverState *bs, const char *filename, int flags,
>               BlockDriver *drv);

How are the errors passed?

Alternative version with file descriptor or struct FILE instead of
filename might become useful but can be added later.

> void qbdrvs_close(BlockDriverState *bs);
> int qbdrvs_img_create(const char *filename, const char *fmt,
>                     const char *base_filename, const char *base_fmt,
>                     char *options, uint64_t img_size, int flags);

'const char *options'

>
>
> /* sync access */
> int qbdrvs_read(BlockDriverState *bs, int64_t sector_num,
>               uint8_t *buf, int nb_sectors);
> int qbdrvs_write(BlockDriverState *bs, int64_t sector_num,
>                const uint8_t *buf, int nb_sectors);

Do we want to use sectors here? How about just raw byte offsets and
number of bytes? I'd leave any hardware details out and just provide
file semantics (open/read/write/close). Future QEMU refactorings could
make supporting HW info inconvenient. If HW details (geometry etc., VM
state) are needed, there should be a separate API.

Vectored I/O might be useful too.

>
>
> /* info retrieve */
> //sector, size and geometry info
> int qbdrvs_get_info(BlockDriverState *bs, BlockDriverInfo *bdi);

Currently BlockDriverInfo does not look very useful for outside users,
with the exception of dirty state. How about accessors instead:
bool qbdrvs_is_dirty(BlockDriverState *bs);

Also the format (QCOW) is needed so that the user can use backing file
functions:
const char *qbdrvs_get_format(BlockDriverState *bs);

> int64_t qbdrvs_getlength(BlockDriverState *bs);
> int64_t qbdrvs_get_allocated_file_size(BlockDriverState *bs);
> void qbdrvs_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr);

Also this function shouldn't be needed if we don't give HW details
with this API.

> //image type
> const char *qbdrvs_get_format_name(BlockDriverState *bs);
> //backing file info
> void qbdrvs_get_backing_filename(BlockDriverState *bs,
>                                char *filename, int filename_size);
> void qbdrvs_get_full_backing_filename(BlockDriverState *bs,
>                                     char *dest, size_t sz);

These are specific to QCOW etc., so
bool qbdrvs_has_backing_files(BlockDriverState *bs)?

>
>
> /* advanced image content access */
> int qbdrvs_is_allocated(BlockDriverState *bs, int64_t sector_num, int
> nb_sectors,
>                       int *pnum);
> int qbdrvs_discard(BlockDriverState *bs, int64_t sector_num, int
> nb_sectors);

Again, some files do not have a concept of allocation.

> int qbdrvs_has_zero_init(BlockDriverState *bs);
>
>
>> Il 16/07/2012 10:16, Wenchao Xia ha scritto:
>>>>
>>>>
>>>    Really thanks for the investigation, I paid quite sometime to dig out
>>> which license is compatible to LGPL, this have sorted it out.
>>>    The coroutine and structure inside is quite a challenge.
>>
>>
>> Coroutines are really just a small complication in the program flow if
>> all you support is synchronous access to files (i.e. no HTTP etc.).
>> Their usage should be completely transparent.
>>
>>> What about
>>> provide the library first in nbd + sync access, and waiting for the
>>> library employer response? If it is good to use, then replace implement
>>> code to native qemu block layer code, change code's license, while keep
>>> API unchanged.
>>
>>
>> You can start by proposing the API.
>>
>> Paolo
>>
>
>
> --
> Best Regards
>
> Wenchao Xia
>
>
>