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

[Peter Lieven]

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?

Peter