Re: [Qemu-devel] [Qemu-block] RFC cdrom in own thread?

[Peter Lieven <pl@kamp.de>]

Am 18.06.2015 um 10:42 schrieb Kevin Wolf:
> Am 18.06.2015 um 10:30 hat Peter Lieven geschrieben:
>> Am 18.06.2015 um 09:45 schrieb Kevin Wolf:
>>> Am 18.06.2015 um 09:12 hat Peter Lieven geschrieben:
>>>> Thread 2 (Thread 0x7ffff5550700 (LWP 2636)):
>>>> #0  0x00007ffff5d87aa3 in ppoll () from /lib/x86_64-linux-gnu/libc.so.6
>>>> No symbol table info available.
>>>> #1  0x0000555555955d91 in qemu_poll_ns (fds=0x5555563889c0, nfds=3,
>>>>      timeout=4999424576) at qemu-timer.c:326
>>>>          ts = {tv_sec = 4, tv_nsec = 999424576}
>>>>          tvsec = 4
>>>> #2  0x0000555555956feb in aio_poll (ctx=0x5555563528e0, blocking=true)
>>>>      at aio-posix.c:231
>>>>          node = 0x0
>>>>          was_dispatching = false
>>>>          ret = 1
>>>>          progress = false
>>>> #3  0x000055555594aeed in bdrv_prwv_co (bs=0x55555637eae0, offset=4292007936,
>>>>      qiov=0x7ffff554f760, is_write=false, flags=0) at block.c:2699
>>>>          aio_context = 0x5555563528e0
>>>>          co = 0x5555563888a0
>>>>          rwco = {bs = 0x55555637eae0, offset = 4292007936,
>>>>            qiov = 0x7ffff554f760, is_write = false, ret = 2147483647, flags = 0}
>>>> #4  0x000055555594afa9 in bdrv_rw_co (bs=0x55555637eae0, sector_num=8382828,
>>>>      buf=0x7ffff44cc800 "(", nb_sectors=4, is_write=false, flags=0)
>>>>      at block.c:2722
>>>>          qiov = {iov = 0x7ffff554f780, niov = 1, nalloc = -1, size = 2048}
>>>>          iov = {iov_base = 0x7ffff44cc800, iov_len = 2048}
>>>> #5  0x000055555594b008 in bdrv_read (bs=0x55555637eae0, sector_num=8382828,
>>>>      buf=0x7ffff44cc800 "(", nb_sectors=4) at block.c:2730
>>>> No locals.
>>>> #6  0x000055555599acef in blk_read (blk=0x555556376820, sector_num=8382828,
>>>>      buf=0x7ffff44cc800 "(", nb_sectors=4) at block/block-backend.c:404
>>>> No locals.
>>>> #7  0x0000555555833ed2 in cd_read_sector (s=0x555556408f88, lba=2095707,
>>>>      buf=0x7ffff44cc800 "(", sector_size=2048) at hw/ide/atapi.c:116
>>>>          ret = 32767
>>> Here is the problem: The ATAPI emulation uses synchronous blk_read()
>>> instead of the AIO or coroutine interfaces. This means that it keeps
>>> polling for request completion while it holds the BQL until the request
>>> is completed.
>> I will look at this.
>>
>>> We can (and should) fix that, otherwise the VCPUs is blocked while we're
>>> reading from the image, even without a hang. It doesn't fully fix your
>>> problem, though, as bdrv_drain_all() and friends still exist.
>> Any idea which commands actually call bdrv_drain_alll?
> At least 'stop' and all commands changing the BDS graph (block jobs,
> snapshots, commit, etc.). For a full list, I would have to inspect each
> command in the code.
>
> The guest can even trigger bdrv_drain_all() by stopping a running DMA
> operation.

Unfortunately, excactly this is happening...
Is there any way to avoid the bdrv_drain_all in bmdma_cmd_writeb?

Peter