Re: [Qemu-devel] [PATCH 5/6] qcow2: Let vmstate call qcow2_co_preadv/pwrite directly

[Eric Blake <eblake@redhat.com>]

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On 06/10/2016 10:05 AM, Kevin Wolf wrote:
> We don't really want to go through the block layer in order to read from
> or write to the vmstate in a qcow2 image. Doing so required a few ugly
> hacks like saving and restoring the old image size (because writing to
> vmstate offsets would increase the image size) or disabling the "reads
> after EOF = zeroes" logic. When calling the right functions directly,
> these hacks aren't necessary any more.
> 
> Note that .bdrv_vmstate_load/save() return 0 instead of the number of
> bytes in case of success now.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/qcow2.c | 24 ++++--------------------
>  1 file changed, 4 insertions(+), 20 deletions(-)
> 

> +++ b/block/qcow2.c
> @@ -2903,36 +2903,20 @@ static int qcow2_save_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
>                                int64_t pos)
>  {
>      BDRVQcow2State *s = bs->opaque;
> -    int64_t total_sectors = bs->total_sectors;
> -    bool zero_beyond_eof = bs->zero_beyond_eof;
> -    int ret;
>  
>      BLKDBG_EVENT(bs->file, BLKDBG_VMSTATE_SAVE);
> -    bs->zero_beyond_eof = false;
> -    ret = bdrv_pwritev(bs, qcow2_vm_state_offset(s) + pos, qiov);
> -    bs->zero_beyond_eof = zero_beyond_eof;
> -
> -    /* bdrv_co_do_writev will have increased the total_sectors value to include
> -     * the VM state - the VM state is however not an actual part of the block
> -     * device, therefore, we need to restore the old value. */
> -    bs->total_sectors = total_sectors;
> -
> -    return ret;
> +    return bs->drv->bdrv_co_pwritev(bs, qcow2_vm_state_offset(s) + pos,
> +                                    qiov->size, qiov, 0);
>  }

bs->drv->bdrv_co_pwritev() is an optional interface; not all the drivers
have it yet.  Should you be asserting that it exists, and/or returning
an error if it does not?

>  
>  static int qcow2_load_vmstate(BlockDriverState *bs, QEMUIOVector *qiov,
>                                int64_t pos)
>  {
>      BDRVQcow2State *s = bs->opaque;
> -    bool zero_beyond_eof = bs->zero_beyond_eof;
> -    int ret;
>  
>      BLKDBG_EVENT(bs->file, BLKDBG_VMSTATE_LOAD);
> -    bs->zero_beyond_eof = false;
> -    ret = bdrv_preadv(bs, qcow2_vm_state_offset(s) + pos, qiov);
> -    bs->zero_beyond_eof = zero_beyond_eof;
> -
> -    return ret;
> +    return bs->drv->bdrv_co_preadv(bs, qcow2_vm_state_offset(s) + pos,
> +                                   qiov->size, qiov, 0);

Ditto.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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