Re: [Qemu-devel] Re: [PATCH 09/16] scsi-disk: Allocate iovec dynamically

[Kevin Wolf <kwolf@redhat.com>]

Am 19.11.2010 13:30, schrieb Hannes Reinecke:
> On 11/19/2010 12:43 PM, Kevin Wolf wrote:
>> Am 18.11.2010 17:28, schrieb Hannes Reinecke:
>>> On 11/18/2010 04:33 PM, Gerd Hoffmann wrote:
>>>>   Hi,
>>>>
>>>>> +static size_t scsi_req_iov_len(SCSIDiskReq *r)
>>>>> +{
>>>>> +    size_t iov_len = 0;
>>>>> +    int i;
>>>>> +
>>>>> +    for (i = 0; i<  r->iov_num; i++)
>>>>> +        iov_len += r->iov[i].iov_len;
>>>>> +
>>>>> +    return iov_len;
>>>>> +}
>>>>
>>>> You are aware that there is a QEMUIOVector type with helper functions
>>>> which keeps track of both number of elements and total size?
>>>>
>>> Yes. But I'm passing passing in an entire iovec to the request.
>>> However, the QEMUIOVector routines allow you to add only _one_
>>> element at a time, which is pretty wasteful here.
>>
>> Does the iov need to be changed afterwards, or why doesn't
>> qemu_iovec_init_external work here?
>>
> Oh, but I _do_ use qemu_iovec_init_external(); cf
> 
> hw/scsi_disk.c:scsi_read_data()
> 
> and yes, the iovec might be modified by the read/write operation;
> see below.
> 
>>> And I'm counting the resulting length of the iovec, which might have
>>> been changed by read/write operations. For which there is no generic
>>> function either.
>>
>> Can you explain which kind of read/write operations would change the
>> iov? This is not completely clear to me.
>>
> It is perfectly valid to send down an iovec larger than the command
> would fill out; eg for a SCSI Inquiry you could easily request 255
> bytes, but the command itself would only provide you with say 36 bytes.

Okay, so at the point where you create the iovec you don't have this
information yet but rather let the command emulation update the iovec
later. Makes sense.

That's probably something that QEMUIOVectors isn't really designed for,
so just forget what I said. :-)

Kevin