[Qemu-devel] Re: [PATCH 3/3] Drop internal bdrv_pread()/bdrv_pwrite() APIs

[Anthony Liguori]

Avi Kivity wrote:
> Now that scsi generic no longer uses bdrv_pread() and bdrv_pwrite(), we can
> drop the corresponding internal APIs, which overlap bdrv_read()/bdrv_write()
> and, being byte oriented, are unnatural for a block device.
>
> Signed-off-by: Avi Kivity <address@hidden>
>
>  int bdrv_truncate(BlockDriverState *bs, int64_t offset)
> diff --git a/block_int.h b/block_int.h
> index e4630f0..cc9966b 100644
> --- a/block_int.h
> +++ b/block_int.h
> @@ -58,10 +58,6 @@ struct BlockDriver {
>      int aiocb_size;
>  
>      const char *protocol_name;
> -    int (*bdrv_pread)(BlockDriverState *bs, int64_t offset,
> -                      uint8_t *buf, int count);
> -    int (*bdrv_pwrite)(BlockDriverState *bs, int64_t offset,
> -                       const uint8_t *buf, int count);
>   

$ grep -l bdrv_pwrite *.c hw/*.c
block.c
block-qcow2.c
block-qcow.c
block-raw-posix.c
block-raw-win32.c
block-vmdk.c
block-vpc.c
savevm.c
hw/scsi-generic.c

So there's a lot of users other than scsi-generic.  Usually, these 
callers are in the block layer to read/write metadata that isn't always 
block aligned.  Some buffer adjustment could fix savevm.c to ensure 
alignment.

These users are now relegated to using emulated pread/pwrite?  Won't 
that have a noticable impact on performance if updating small bits of 
metadata.  For instance, I think updating qcow2 refcounts would look bad 
since you have to read/write the full block to update 4 bytes of data.  
Granted it'll be cached, but...

Regards,

Anthony Liguori
>      int (*bdrv_truncate)(BlockDriverState *bs, int64_t offset);
>      int64_t (*bdrv_getlength)(BlockDriverState *bs);
>      int (*bdrv_write_compressed)(BlockDriverState *bs, int64_t sector_num,
>