Re: [Qemu-devel] questions about the bdrv_co_do_readv()

[Yaodong Yang]

I checked the argument, it looks like the same. 

I guess my confusion is the follows:

code version: qemu-1.5.1

step 1:
 file:  block-migration.c, 
 function: static int mig_save_device_bulk(QEMUFile *f, BlkMigDevState *bmds)

     Inside this function, qemu calls the following function to read a chunk of 
data for migration.
blk->aiocb = bdrv_aio_readv(bs, cur_sector, &blk->qiov,
                                nr_sectors, blk_mig_read_cb, blk);
my questions:
No


step 2:
file block.c
function: BlockDriverAIOCB *bdrv_aio_readv(BlockDriverState *bs, int64_t 
sector_num,
                                 QEMUIOVector *qiov, int nb_sectors,
                                 BlockDriverCompletionFunc *cb, void *opaque)
my questions:
No

Step 3:
file block.c
function：
static BlockDriverAIOCB *bdrv_co_aio_rw_vector(BlockDriverState *bs,
                                               int64_t sector_num,
                                               QEMUIOVector *qiov,
                                               int nb_sectors,
                                               BlockDriverCompletionFunc *cb,
                                               void *opaque,
                                               bool is_write)
This function create a coroutine and then enter this coroutine. 
    co = qemu_coroutine_create(bdrv_co_do_rw);
    qemu_coroutine_enter(co, acb);

My questions:
1. What is the purpose of creating a coroutine here? 



Step 4:
file: block.c
function: static void coroutine_fn bdrv_co_do_rw(void *opaque)
In this function, the bdrv_co_do_readv() is called. and After it, it create a 
bottom-half and then schedule this buttom half.
acb->req.error = bdrv_co_do_readv(bs, acb->req.sector,
            acb->req.nb_sectors, acb->req.qiov, 0);

My questions:
1. is the bdrv_co_do_readv function called within the IOthread or Migration 
thread?

Step 5
file: block.c
function: static void bdrv_co_em_bh(void *opaque)

My questions:
1. Is this function called within the IOthread or Migration thread?

In sum,
I do not know clearly how the coroutine, bottom-half schemes are used for the 
migration job. is there any reference material related?

Thanks a lot !
Yaodong 








it send s the bdrv_co_do_readv(block.c) function is exectued 


On Aug 27, 2013, at 2:45 AM, Stefan Hajnoczi <stefa...@gmail.com> wrote:

> On Mon, Aug 26, 2013 at 09:59:51PM -0500, Yaodong Yang wrote:
>> In sum, the bdrv_co_do_readv() seems to be executed inside two thread, the 
>> migration thread and native iothread. Both of them executed the function 
>> twice for a single request. Could someone explain it for me ? I appreciate 
>> it very much!
> 
> Did you check the bdrv_co_readv(bs=...) argument?  The calls may be
> referring to two different BlockDriverState instances.
> 
> A raw image file looks like this:
> 
> BDS#1 (block/raw.c)
>   ->backing_hd = NULL
>   ->file = BDS#2
> 
> BDS#2 (block/raw-posix.c)
> 
> With qcow2 and no backing file you would have something like this:
> 
> BDS#1 (block/qcow2.c)
>   ->backing_hd = NULL
>   ->file = BDS#2
> 
> BDS#2 (block/raw-posix.c)
> 
> Expect to see calls forwarded from BDS#1 to BDS#2.
> 
> Stefan