Re: [Qemu-devel] [PATCH 1/5] ide/atapi: make PIO read requests async

[Peter Lieven <pl@kamp.de>]

Am 06.10.2015 um 19:56 schrieb John Snow:
>
> On 10/06/2015 01:12 PM, Peter Lieven wrote:
>>> Am 06.10.2015 um 19:07 schrieb John Snow <jsnow@redhat.com>:
>>>
>>>
>>>
>>>> On 10/06/2015 05:20 AM, Peter Lieven wrote:
>>>>> Am 06.10.2015 um 10:57 schrieb Kevin Wolf:
>>>>> Am 05.10.2015 um 23:15 hat John Snow geschrieben:
>>>>>> On 09/21/2015 08:25 AM, Peter Lieven wrote:
>>>>>>> PIO read requests on the ATAPI interface used to be sync blk requests.
>>>>>>> This has to siginificant 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.
>>>>>>>
>>>>>>> Signed-off-by: Peter Lieven <pl@kamp.de>
>>>>>>> ---
>>>>>>>  hw/ide/atapi.c | 69
>>>>>>> ++++++++++++++++++++++++++++++++++++----------------------
>>>>>>>  1 file changed, 43 insertions(+), 26 deletions(-)
>>>>>>>
>>>>>>> diff --git a/hw/ide/atapi.c b/hw/ide/atapi.c
>>>>>>> index 747f466..9257e1c 100644
>>>>>>> --- a/hw/ide/atapi.c
>>>>>>> +++ b/hw/ide/atapi.c
>>>>>>> @@ -105,31 +105,51 @@ 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)
>>>>>>>  {
>>>>>>> -    int ret;
>>>>>>> +    IDEState *s = opaque;
>>>>>>>  -    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:
>>>>>>> -        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;
>>>>>>> +    block_acct_done(blk_get_stats(s->blk), &s->acct);
>>>>>>> +
>>>>>>> +    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);
>>>>>>>      }
>>>>>>> -    return ret;
>>>>>>> +
>>>>>>> +    s->lba++;
>>>>>>> +    s->io_buffer_index = 0;
>>>>>>> +    s->status &= ~BUSY_STAT;
>>>>>>> +
>>>>>>> +    ide_atapi_cmd_reply_end(s);
>>>>>>> +}
>>>>>>> +
>>>>>>> +static int cd_read_sector(IDEState *s, int lba, void *buf, int
>>>>>>> sector_size)
>>>>>>> +{
>>>>>>> +    if (sector_size != 2048 && sector_size != 2352) {
>>>>>>> +        return -EINVAL;
>>>>>>> +    }
>>>>>>> +
>>>>>>> +    s->iov.iov_base = buf;
>>>>>>> +    if (sector_size == 2352) {
>>>>>>> +        buf += 4;
>>>>>>> +    }
>>>>> This doesn't look quite right, buf is never read after this.
>>>>>
>>>>> Also, why +=4 when it was originally buf + 16?
>>>> You are right. I mixed that up.
>>>>
>>>>>>> +
>>>>>>> +    s->iov.iov_len = 4 * BDRV_SECTOR_SIZE;
>>>>>>> +    qemu_iovec_init_external(&s->qiov, &s->iov, 1);
>>>>>>> +
>>>>>>> +    if (blk_aio_readv(s->blk, (int64_t)lba << 2, &s->qiov, 4,
>>>>>>> +                      cd_read_sector_cb, s) == NULL) {
>>>>>>> +        return -EIO;
>>>>>>> +    }
>>>>>>> +
>>>>>>> +    block_acct_start(blk_get_stats(s->blk), &s->acct,
>>>>>>> +                     4 * BDRV_SECTOR_SIZE, BLOCK_ACCT_READ);
>>>>>>> +    s->status |= BUSY_STAT;
>>>>>>> +    return 0;
>>>>>>>  }
>>>>>> We discussed this off-list a bit, but for upstream synchronization:
>>>>>>
>>>>>> Unfortunately, I believe making cd_read_sector here non-blocking makes
>>>>>> ide_atapi_cmd_reply_end non-blocking, and as a result makes calls to
>>>>>> s->end_transfer_func() nonblocking, which functions like ide_data_readw
>>>>>> are not prepared to cope with.
>>>>> I don't think that's a problem as long as BSY is set while the
>>>>> asynchronous command is running and DRQ is cleared. The latter will
>>>>> protect ide_data_readw(). ide_sector_read() does essentially the same
>>>>> thing.
>>>> I was thinking the same. Without the BSY its not working at all.
>>>>
>>>>> Or maybe I'm just missing what you're trying to say.
>>>>>
>>>>>> My suggestion is to buffer an entire DRQ block of data at once
>>>>>> (byte_count_limit) to avoid the problem.
>>>>> No matter whether there is a problem or not, buffering more data at once
>>>>> (and therefore doing less requests) is better for performance anyway.
>>>> Its possible to do only one read in the backend and read the whole
>>>> request into the IO buffer. I send a follow-up.
>>> Be cautious: we only have 128K (+4 bytes) to play with in the io_buffer
>>> and the READ10 cdb can request up to 128MiB! For performance, it might
>>> be nice to always buffer something like:
>>>
>>> MIN(128K, nb_sectors * sector_size)
>> isnt nb_sectors limited to CD_MAX_SECTORS (32)?
>>
>> Peter
>>
> CD_MAX_SECTORS is... (80 * 60 * 75 * 2048) / 512 --> 1440000, and
> describes the maximum sector size for a CD medium, not the request size.
>
> Where'd you get the 32 number?

You are right. I mixed this up. You where talking of a maximum transfer
size of close to 32 sectors. But you where referring to an ide transfer not
the maximum request size in terms of SCSI block limits.

I will rework that patch on Thursday.

Maybe you can have a look at the other patches of this series as well? Then I can
respin the whole series.

Thanks for your help,
Peter