kvm-64 hangs under heavy disk I/O with scsi disks.  To reproduce,
create a fresh qcow2 disk, boot linux, and execute

  dd if=/dev/sdX of=/dev/null bs=1M

on the fresh disk.  See also https://sourceforge.net/tracker/?func=detail&atid=893831&aid=1895893&group_id=180599

I have attached a patch that appears to fix the problem.  The bug
seems to be the following.  scsi_read_data() does the following

    bdrv_aio_read()
    r->sector += n;
    r->sector_count -= n;

For reasons that I do not fully understand, bdrv_aio_read() does
not return immediately, but instead it calls scsi_read_data()
recursively.  Since ``r->sector += n;'' has not been executed
yet, the re-entrant call triggers a read of the same sector, which
breaks the producer-consumer lockstep.  The fix is to swap the operations
as follows:

    r->sector += n;
    r->sector_count -= n;
    bdrv_aio_read()

A similar fix applies to scsi_write_data().

Thanks for developing kvm, it is truly an amazing piece of software.

Regards,
Matteo Frigo