Re: [Qemu-devel] [Block dev] : Qemu block ide_dma_read call routine

[Shailesh Kumar <tsaileshkumar@gmail.com>]

On Mon, Feb 23, 2015 at 3:25 AM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 11.02.2015 um 04:51 hat Shailesh Kumar geschrieben:
>> Hi,
>>
>>     I am implementing read equivalent routine in qemu. Can some one
>> help me understand  control flow of the qemu read/write
>> implementation.
>>
>> I am using xen-4.2.0 and qemu-1.6.1
>>
>> My requirement is simple:
>>
>> I have a 1024*1024 buffer already filled with some useful data.
>>
>> Now when windows (my guest OS) does IDE_DMA_READ command to the disk,
>> I want to intercept it and fill data from my private buffer.
>>
>> my intention is to leverage existing dma_read infrastructure and
>> overwrite the read buffer-data at the lowest level of qemu . That way
>> the buffers /vectors "qiov" which are prepared due to cmd IDE_DMA_READ
>> will copy and return data from my data-buffer to guest-OS.
>>
>> I could trace the control from.
>>
>> ide_sector_start_dma
>> -> s->bus->dma->ops->start_dma
>>     -> ide_dma_cb
>>         ->dma_bdrv_read
>>            -> bdrv_aio_readv
>> .              ->bdrv_co_aio_rw_vector
>>                   -> bdrv_co_do_rw   "coroutine"
>>                       -> bdrv_co_do_readv
>>                           -> drv->bdrv_co_readv (( in my case it is
>> from raw.c raw_co_readv ))
>>                              -> bdrv_co_readv
>
> You need to be aware that BlockDriverStates are actually stacked. In
> most common cases you have one protocol driver that accesses the image
> file (this might be file, nbd, gluster, http, ssh, etc.) and on top of
> that a driver that interprets the image format (like raw, qcow2, vmdk,
> etc.)
>
> You stopped your call chain at the point where the request has passed
> through the 'raw' format driver. This final bdrv_co_readv() calls into
> drv->bdrv_co_readv of the 'file' driver in raw-posix.c, which is doing
> the actual syscalls.
>
>>                       -> bdrv_co_do_readv
>>
>>                    ->in bdrv_co_do_rw  the bottom half is scheduled
>> bdrv_co_em_bh -->> this will invoke     -> ide_dma_cb () which is
>> again the starting point. Looks like there a double-linked list
>> maintained for the coroutine entries and are off loaded to qemu-wait
>> queue during this process.
>>
>> Now I need help to understand where to look for to find the last
>> read/write system call which will get the data out from the disk for
>> guest-OS (windows) .
>>
>> I am seeking suggestions and help for the same.
>
> As it might already have occurred to you reading the above, the stacking
> provides you a clean way to implement your interception. You just need to
> insert a filtering BlockDriver in the chain, so that you end up with:
>
> raw -> intercept -> file
>
> You could have a look at the quorum or blkdebug drivers, which can be
> inserted in the same way. In contrast to those two drivers, I'd
> recommend you to implement bdrv_co_readv/writev instead of
> bdrv_aio_readv/writev, because they are probably simpler to use for your
> case.
>
> Kevin

Kevin,

Thanks for the detailed explanation.
Your email gave me little more insight about the control flow.

Gmail has put this email at different place and I was not able to find
it till now. hence delayed reply.

I am confused with format_names.
What does host_device mean and what does raw means ?
drv->format_name does it mean what form of the driver to be used ?
what attributes decide which driver to be used.

for e.g: if I print the drv->format_name in bdrv_co_readv I see raw
and host_device being printed alternately.

-Sailesh