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

[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.

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

> diff --git a/hw/ide/atapi.c b/hw/ide/atapi.c
> index 950e311..cdcbd49 100644
> --- a/hw/ide/atapi.c
> +++ b/hw/ide/atapi.c
> @@ -27,6 +27,8 @@
>  #include "hw/scsi/scsi.h"
>  #include "sysemu/block-backend.h"
>
> +#define DEBUG_IDE_ATAPI 1
> +
>  static void ide_atapi_cmd_read_dma_cb(void *opaque, int ret);
>
>  static void padstr8(uint8_t *buf, int buf_size, const char *src)
> @@ -105,31 +107,55 @@ static void cd_data_to_raw(uint8_t *buf, int lba)
>      memset(buf, 0, 288);
>  }
>
> -static int cd_read_sector(IDEState *s, int lba, uint8_t *buf, int 
> sector_size)
> +static void cd_read_sector_cb(void *opaque, int ret) {
> +    IDEState *s = opaque;
> +
> +    block_acct_done(blk_get_stats(s->blk), &s->acct);
> +
> +    printf("cd_read_sector_cb lba %d ret = %d\n", s->lba, ret);
> +
> +    if (ret < 0) {
> +        ide_atapi_io_error(s, ret);
> +        return;
> +    }
> +
> +    if (s->cd_sector_size == 2352) {
> +        cd_data_to_raw(s->io_buffer, s->lba);
> +    }
> +
> +    s->lba++;
> +    s->io_buffer_index = 0;
> +
> +    ide_atapi_cmd_reply_end(s);
> +}
> +
> +static BlockAIOCB *cd_read_sector(IDEState *s, int lba, void *buf, int 
> sector_size)
>  {
> -    int ret;
> +    BlockAIOCB *aiocb = NULL;
>
> -    switch(sector_size) {
> -    case 2048:
> -        block_acct_start(blk_get_stats(s->blk), &s->acct,
> -                         4 * BDRV_SECTOR_SIZE, BLOCK_ACCT_READ);
> -        ret = blk_read(s->blk, (int64_t)lba << 2, buf, 4);
> -        block_acct_done(blk_get_stats(s->blk), &s->acct);
> -        break;
> -    case 2352:
> +    if (sector_size != 2048 && sector_size != 2352) {
> +        return NULL;
> +    }
> +
> +    s->iov.iov_base = buf;
> +    if (sector_size == 2352) {
> +        buf += 4;
> +    }
> +
> +    s->iov.iov_len = 4 * BDRV_SECTOR_SIZE;
> +    qemu_iovec_init_external(&s->qiov, &s->iov, 1);
> +
> +    printf("cd_read_sector lba %d\n", lba);
> +
> +    aiocb = blk_aio_readv(s->blk, (int64_t)lba << 2, &s->qiov, 4,
> +                          cd_read_sector_cb, s);
> +
> +    if (aiocb != NULL) {
>          block_acct_start(blk_get_stats(s->blk), &s->acct,
>                           4 * BDRV_SECTOR_SIZE, BLOCK_ACCT_READ);
> -        ret = blk_read(s->blk, (int64_t)lba << 2, buf + 16, 4);
> -        block_acct_done(blk_get_stats(s->blk), &s->acct);
> -        if (ret < 0)
> -            return ret;
> -        cd_data_to_raw(buf, lba);
> -        break;
> -    default:
> -        ret = -EIO;
> -        break;
>      }
> -    return ret;
> +
> +    return aiocb;
>  }
>
>  void ide_atapi_cmd_ok(IDEState *s)
> @@ -170,7 +196,7 @@ void ide_atapi_io_error(IDEState *s, int ret)
>  /* The whole ATAPI transfer logic is handled in this function */
>  void ide_atapi_cmd_reply_end(IDEState *s)
>  {
> -    int byte_count_limit, size, ret;
> +    int byte_count_limit, size;
>  #ifdef DEBUG_IDE_ATAPI
>      printf("reply: tx_size=%d elem_tx_size=%d index=%d\n",
>             s->packet_transfer_size,
> @@ -187,13 +213,10 @@ void ide_atapi_cmd_reply_end(IDEState *s)
>      } else {
>          /* see if a new sector must be read */
>          if (s->lba != -1 && s->io_buffer_index >= s->cd_sector_size) {
> -            ret = cd_read_sector(s, s->lba, s->io_buffer, s->cd_sector_size);
> -            if (ret < 0) {
> -                ide_atapi_io_error(s, ret);
> -                return;
> +            if (cd_read_sector(s, s->lba, s->io_buffer, s->cd_sector_size) 
> == NULL) {
> +                ide_atapi_io_error(s, -EIO);
>              }
> -            s->lba++;
> -            s->io_buffer_index = 0;
> +            return;
>          }
>          if (s->elementary_transfer_size > 0) {
>              /* there are some data left to transmit in this elementary
>
> Thanks,
> Peter