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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC] introduce a dynamic library to expose qemu block API
Date: Wed, 18 Jul 2012 11:03:23 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120615 Thunderbird/13.0.1

Il 18/07/2012 10:51, Wenchao Xia ha scritto:
>   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.

block_int.h is BSD-licensed.  It's compatible.

>   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.

I'll note that img_create, and almost all retrieval functions cannot be
implemented on top of NBD.

> /* structure init and uninit */
> BlockDriverState *qbdrvs_new(const char *device_name);

device_name is not needed in the library.

> void qbdrvs_delete(BlockDriverState *bs);

Being able to reuse the same BDS with close/open is one of the worst
parts of the QEMU block layer.  It is only needed to implement eject in
QEMU.  Let's get things right, and only have open/close:

int qbdrvs_open(BlockDriverState **bs, const char *filename, int flags,
                const char *format_name);
void qbdrvs_close(BlockDriverState *bs);

> /* file open and close */
> int qbdrvs_open(BlockDriverState *bs, const char *filename, int flags,
>               BlockDriver *drv);

You didn't have an API to find the BlockDriver *.  Let's just pass a
format name, or NULL for probing (see above). This is consistent with

> 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);
> /* 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);

I would like to have also a scatter gather API (qbdrvs_readv and
qbdrvs_writev) taking a "struct iovec *iov, int niov" instead of
"uint8_t *buf, int nb_sectors".

flush is missing.

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

One problem here is that BlockDriverInfo may grow in the future.  This
is a problem for the ABI of the library.  I don't have any particular
ideas here, except adding a separate API to the library for each member
of BlockDriverInfo.

> 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);

Not needed, it's the same as bdrv_getlength.

> //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);
> /* 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);
> int qbdrvs_has_zero_init(BlockDriverState *bs);


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