Re: [Qemu-devel] [PATCHv4 3/4] block/iscsi: set max_transfer_length

[Max Reitz <mreitz@redhat.com>]

On 2014-10-25 at 17:16, Peter Lieven wrote:
> Am 23.10.2014 um 13:18 schrieb Max Reitz:
>> On 2014-10-16 at 09:54, Peter Lieven wrote:
>>> the limit of 0xffffff for 16 byte CDBs is intentional to
>>> avoid overflows on 32-bit architectures.
>> How is it related to 32 bit? I somehow feel like it has to do something with the result of sector_lun2qemu() which involves block_size...
> iscsilun->bl.max_xfer_len is 32-bit unsigned while nb_sectors is usually signed. Furthermore as you suspected nb_sectors is always 512Byte sectors
> while iscsilun->block_size can be 4k or even more.
>
> I will change the code to set max_xfer_len to 0xffffffff and limit the output of sector_lun2qemu() to INT_MAX in the bs->bl.max_transfer_length case.
>
> However, in real life you will never want to have a transfer of 0x3fffffff blocks, won't you?

I'd be fine with a single block, but QEMU guests matter more than me. :-)

I was just wondering how you ended up with 2^24 - 1 as it would be (2^32 
- 1) / 256. I do remember IDE doing 16-bit-transfers, so I could imagine 
how this works out with 512 byte sector size, but this isn't IDE, so...

(sorry, it's been a while since I worked with the SCSI protocol myself)

Max

> Peter
>
>
>>> Signed-off-by: Peter Lieven <pl@kamp.de>
>>> Reviewed-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
>>> ---
>>>    block/iscsi.c |   12 ++++++++++--
>>>    1 file changed, 10 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/block/iscsi.c b/block/iscsi.c
>>> index 3a01de0..c873d13 100644
>>> --- a/block/iscsi.c
>>> +++ b/block/iscsi.c
>>> @@ -1449,10 +1449,18 @@ static void iscsi_close(BlockDriverState *bs)
>>>      static void iscsi_refresh_limits(BlockDriverState *bs, Error **errp)
>>>    {
>>> -    IscsiLun *iscsilun = bs->opaque;
>>> -
>>>        /* We don't actually refresh here, but just return data queried in
>>>         * iscsi_open(): iscsi targets don't change their limits. */
>>> +
>>> +    IscsiLun *iscsilun = bs->opaque;
>>> +    uint32_t max_xfer_len = iscsilun->use_16_for_rw ? 0xffffff : 0xffff;
>>> +
>>> +    if (iscsilun->bl.max_xfer_len) {
>>> +        max_xfer_len = MIN(max_xfer_len, iscsilun->bl.max_xfer_len);
>>> +    }
>>> +
>>> +    bs->bl.max_transfer_length = sector_lun2qemu(max_xfer_len, iscsilun);
>>> +
>>>        if (iscsilun->lbp.lbpu) {
>>>            if (iscsilun->bl.max_unmap < 0xffffffff) {
>>>                bs->bl.max_discard = sector_lun2qemu(iscsilun->bl.max_unmap,