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Description of problem:
Just as discus with Paolo in IRC, we should add support cache=none to -drive for scsi-generic device in rhel7 as libvirt supports cache="".
If i boot guest with the cache=none for scsi-generic drive, it will fail to boot up. btw, the "writeback", "unsafe", "directsync" and "writethrough" is ok for cache value of scsi-generic drive. 

Version-Release number of selected component (if applicable):
host info:
kernel-3.8.0-0.40.el7.x86_64
qemu-kvm-1.4.0-1.el7.x86_64
guest info:
kernel-3.8.0-0.40.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.boot guest with scsi-generic adding cache=none.
e.g:...-drive file=/dev/sg2,if=none,id=drive-data-disk,format=raw,cache=none,aio=native -device virtio-scsi-pci,id=scsi1,addr=0x5 -device scsi-generic,bus=scsi1.0,drive=drive-data-disk,id=data-disk 
2.
3.
  
Actual results:
after step 1, it will fail to boot up and qemu prompt like:
qemu-kvm: -drive file=/dev/sg2,if=none,id=drive-data-disk,format=raw,cache=none,aio=native: could not open disk image /dev/sg2: Invalid argument

Expected results:
we should add support for cache=none to scsi-generic.

Additional info:

The libvirt patches for scsi-generic do not provide a way to specify the cache= argument, which makes sense since scsi-generic is really always behaving as if you specified cache=none.  I'm closing this bug.