Re: [Qemu-devel] [PATCH 09/10] posix-aio-compat: Allow read after EOF

[Kevin Wolf <kwolf@redhat.com>]

Am 26.07.2011 15:55, schrieb Frediano Ziglio:
> 2011/7/26 Kevin Wolf <kwolf@redhat.com>:
>> In order to be able to transparently replace bdrv_read calls by bdrv_co_read,
>> reading beyond EOF must produce zeros instead of short reads for AIO, too.
>>
>> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
>> ---
>>  posix-aio-compat.c |   19 +++++++++++++++++++
>>  1 files changed, 19 insertions(+), 0 deletions(-)
>>
>> diff --git a/posix-aio-compat.c b/posix-aio-compat.c
>> index 788d113..8dc00cb 100644
>> --- a/posix-aio-compat.c
>> +++ b/posix-aio-compat.c
>> @@ -198,6 +198,12 @@ static ssize_t handle_aiocb_rw_vector(struct qemu_paiocb *aiocb)
>>     return len;
>>  }
>>
>> +/*
>> + * Read/writes the data to/from a given linear buffer.
>> + *
>> + * Returns the number of bytes handles or -errno in case of an error. Short
>> + * reads are only returned if the end of the file is reached.
>> + */
>>  static ssize_t handle_aiocb_rw_linear(struct qemu_paiocb *aiocb, char *buf)
>>  {
>>     ssize_t offset = 0;
>> @@ -334,6 +340,19 @@ static void *aio_thread(void *unused)
>>
>>         switch (aiocb->aio_type & QEMU_AIO_TYPE_MASK) {
>>         case QEMU_AIO_READ:
>> +            ret = handle_aiocb_rw(aiocb);
>> +            if (ret >= 0 && ret < aiocb->aio_nbytes && aiocb->common.bs->growable) {
>> +                /* A short read means that we have reached EOF. Pad the buffer
>> +                 * with zeros for bytes after EOF. */
>> +                QEMUIOVector qiov;
>> +
>> +                qemu_iovec_init_external(&qiov, aiocb->aio_iov,
>> +                                         aiocb->aio_niov);
>> +                qemu_iovec_memset_skip(&qiov, 0, aiocb->aio_nbytes - ret, ret);
>> +
>> +                ret = aiocb->aio_nbytes;
>> +            }
>> +            break;
>>         case QEMU_AIO_WRITE:
>>             ret = handle_aiocb_rw(aiocb);
>>             break;
>> --
>> 1.7.6
>>
> 
> Still not tested but I think to know what does it solve :)
> 
> I think Linux AIO require same attention.

In theory yes, but it's not as easy and for some reason I couldn't
reproduce it with Linux AIO (maybe the problematic requests are
misaligned so that it falls back to posix-aio-compat.c), so I decided to
ignore it for now.

Patches are welcome. ;-)

Kevin