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

[Max Reitz]

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 <address@hidden>
> > > Reviewed-by: Ronnie Sahlberg <address@hidden>
> > > ---
> > >    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,