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


From: Peter Lieven
Subject: Re: [Qemu-block] RFC cdrom in own thread?
Date: Mon, 22 Jun 2015 15:09:58 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

Am 22.06.2015 um 11:25 schrieb Stefan Hajnoczi:
On Fri, Jun 19, 2015 at 2:14 PM, Peter Lieven <address@hidden> wrote:
Am 18.06.2015 um 11:36 schrieb Stefan Hajnoczi:
On Thu, Jun 18, 2015 at 10:29 AM, Peter Lieven <address@hidden> wrote:
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.
I need some further help. My way to "emulate" a hung NFS Server is to
block it in the Firewall. Currently I face the problem that I cannot mount
a CD Iso via libnfs (nfs://) without hanging Qemu (i previously tried with
a kernel NFS mount). It reads a few sectors and then stalls (maybe another
bug):

(gdb) thread apply all bt full

Thread 3 (Thread 0x7ffff0c21700 (LWP 29710)):
#0  qemu_cond_broadcast (address@hidden) at
util/qemu-thread-posix.c:120
         err = <optimized out>
         __func__ = "qemu_cond_broadcast"
#1  0x0000555555911164 in rfifolock_unlock (address@hidden) at
util/rfifolock.c:75
         __PRETTY_FUNCTION__ = "rfifolock_unlock"
#2  0x0000555555875921 in aio_context_release (address@hidden)
at async.c:329
No locals.
#3  0x000055555588434c in aio_poll (address@hidden,
address@hidden) at aio-posix.c:272
         node = <optimized out>
         was_dispatching = false
         i = <optimized out>
         ret = <optimized out>
         progress = false
         timeout = 611734526
         __PRETTY_FUNCTION__ = "aio_poll"
#4  0x00005555558bc43d in bdrv_prwv_co (address@hidden,
address@hidden, address@hidden,
address@hidden, address@hidden(unknown: 0)) at
block/io.c:552
         aio_context = 0x5555562598b0
         co = <optimized out>
         rwco = {bs = 0x55555627c0f0, offset = 7038976, qiov =
0x7ffff0c208f0, is_write = false, ret = 2147483647, flags = (unknown: 0)}
#5  0x00005555558bc533 in bdrv_rw_co (bs=0x55555627c0f0,
address@hidden, address@hidden "(",
address@hidden, address@hidden,
     address@hidden(unknown: 0)) at block/io.c:575
         qiov = {iov = 0x7ffff0c208e0, niov = 1, nalloc = -1, size = 2048}
         iov = {iov_base = 0x555557874800, iov_len = 2048}
#6  0x00005555558bc593 in bdrv_read (bs=<optimized out>,
address@hidden, address@hidden "(",
address@hidden) at block/io.c:583
No locals.
#7  0x00005555558af75d in blk_read (blk=<optimized out>,
address@hidden, address@hidden "(",
address@hidden) at block/block-backend.c:493
         ret = <optimized out>
#8  0x00005555557abb88 in cd_read_sector (sector_size=<optimized out>,
buf=0x555557874800 "(", lba=3437, s=0x55555760db70) at hw/ide/atapi.c:116
         ret = <optimized out>
#9  ide_atapi_cmd_reply_end (s=0x55555760db70) at hw/ide/atapi.c:190
         byte_count_limit = <optimized out>
         size = <optimized out>
         ret = 2
This is still the same scenario Kevin explained.

The ATAPI CD-ROM emulation code is using synchronous blk_read().  This
function holds the QEMU global mutex while waiting for the I/O request
to complete.  This blocks other vcpu threads and the main loop thread.

The solution is to convert the CD-ROM emulation code to use
blk_aio_readv() instead of blk_read().
I tried a little, but i am stuck with my approach. I reads one sector
and then doesn't continue. Maybe someone with more knowledge
of ATAPI/IDE could help?
Converting synchronous code to asynchronous requires an understanding
of the device's state transitions.  Asynchronous code has to put the
device registers into a busy state until the request completes.  It
also needs to handle hardware register accesses that occur while the
request is still pending.

That was my assumption as well. But I don't know how to proceed...


I don't know ATAPI/IDE code well enough to suggest a fix.

Maybe @John can help?

Peter



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