Re: [Qemu-devel] [PATCH V3 1/6] ide/atapi: make PIO read requests async

[John Snow <jsnow@redhat.com>]

On 11/06/2015 03:42 AM, Peter Lieven wrote:
> PIO read requests on the ATAPI interface used to be sync blk requests.
> This has two significant drawbacks. First the main loop hangs util an
> I/O request is completed and secondly if the I/O request does not
> complete (e.g. due to an unresponsive storage) Qemu hangs completely.
> 
> Note: Due to possible race conditions requests during an ongoing
> elementary transfer are still sync.
> 
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  hw/ide/atapi.c | 97 ++++++++++++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 85 insertions(+), 12 deletions(-)
> 
> diff --git a/hw/ide/atapi.c b/hw/ide/atapi.c
> index 747f466..29fd131 100644
> --- a/hw/ide/atapi.c
> +++ b/hw/ide/atapi.c
> @@ -105,33 +105,98 @@ 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 int
> +cd_read_sector_sync(IDEState *s, uint8_t *buf)
>  {
>      int ret;
>  
> -    switch(sector_size) {
> +#ifdef DEBUG_IDE_ATAPI
> +    printf("cd_read_sector_sync: lba=%d\n", s->lba);
> +#endif
> +
> +    switch (s->cd_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);
> +        ret = blk_read(s->blk, (int64_t)s->lba << 2, buf, 4);
>          block_acct_done(blk_get_stats(s->blk), &s->acct);
>          break;
>      case 2352:
>          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);
> +        ret = blk_read(s->blk, (int64_t)s->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);
> +        cd_data_to_raw(buf, s->lba);
>          break;
>      default:
>          ret = -EIO;
>          break;
>      }
> +
> +    if (!ret) {
> +        s->lba++;
> +        s->io_buffer_index = 0;
> +    }
> +
>      return ret;
>  }
>  
> +static void cd_read_sector_cb(void *opaque, int ret)
> +{
> +    IDEState *s = opaque;
> +
> +    block_acct_done(blk_get_stats(s->blk), &s->acct);
> +
> +#ifdef DEBUG_IDE_ATAPI
> +    printf("cd_read_sector_cb: lba=%d ret=%d\n", s->lba, ret);
> +#endif
> +
> +    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;
> +    s->status &= ~BUSY_STAT;
> +
> +    ide_atapi_cmd_reply_end(s);
> +}
> +
> +static int cd_read_sector(IDEState *s, void *buf)
> +{
> +    if (s->cd_sector_size != 2048 && s->cd_sector_size != 2352) {
> +        return -EINVAL;
> +    }
> +
> +    s->iov.iov_base = buf;
> +    if (s->cd_sector_size == 2352) {
> +        buf += 16;
> +    }
> +
> +    s->iov.iov_len = 4 * BDRV_SECTOR_SIZE;
> +    qemu_iovec_init_external(&s->qiov, &s->iov, 1);
> +
> +#ifdef DEBUG_IDE_ATAPI
> +    printf("cd_read_sector: lba=%d\n", s->lba);
> +#endif
> +
> +    block_acct_start(blk_get_stats(s->blk), &s->acct,
> +                     4 * BDRV_SECTOR_SIZE, BLOCK_ACCT_READ);
> +
> +    blk_aio_readv(s->blk, (int64_t)s->lba << 2, &s->qiov, 4,
> +                  cd_read_sector_cb, s);
> +
> +    s->status |= BUSY_STAT;
> +    return 0;
> +}
> +
>  void ide_atapi_cmd_ok(IDEState *s)
>  {
>      s->error = 0;
> @@ -182,18 +247,27 @@ void ide_atapi_cmd_reply_end(IDEState *s)
>          ide_atapi_cmd_ok(s);
>          ide_set_irq(s->bus);
>  #ifdef DEBUG_IDE_ATAPI
> -        printf("status=0x%x\n", s->status);
> +        printf("end of transfer, status=0x%x\n", s->status);
>  #endif
>      } 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);
> +            if (!s->elementary_transfer_size) {
> +                ret = cd_read_sector(s, s->io_buffer);
> +                if (ret < 0) {
> +                    ide_atapi_io_error(s, ret);
> +                }
>                  return;
> +            } else {
> +                /* rebuffering within an elementary transfer is
> +                 * only possible with a sync request because we
> +                 * end up with a race condition otherwise */
> +                ret = cd_read_sector_sync(s, s->io_buffer);
> +                if (ret < 0) {
> +                    ide_atapi_io_error(s, ret);
> +                    return;
> +                }
>              }
> -            s->lba++;
> -            s->io_buffer_index = 0;
>          }
>          if (s->elementary_transfer_size > 0) {
>              /* there are some data left to transmit in this elementary
> @@ -275,7 +349,6 @@ static void ide_atapi_cmd_read_pio(IDEState *s, int lba, int nb_sectors,
>      s->io_buffer_index = sector_size;
>      s->cd_sector_size = sector_size;
>  
> -    s->status = READY_STAT | SEEK_STAT;
>      ide_atapi_cmd_reply_end(s);
>  }
>  
> 

Reviewed-by: John Snow <jsnow@redhat.com>

Stefan, do you have time to re-review the latter portion of this series
before hard freeze? I'd like to get this series in before rc0 if at all
possible, to benefit from all the RC testing.

--js