Re: [LSF/MM TOPIC] Target fabric module for SCSI para-virtualization

[Boaz Harrosh <openosd@gmail.com>]

On 02/07/2011 09:57 AM, Nicholas A. Bellinger wrote:
> Greetings all,
> 
> [Topic]
> 
> A hybrid target fabric module for QEMU/KVM SCSI para-virtualization
> 
> [Abstract]
> 
> Currently we use the TCM_Loop v4 fabric module with mainline .38 target
> code to present SCSI LUNs with high level multi-fabric (iSCSI, FC, SAS)
> SPC-3 port WWN emulation into KVM guests using two different forms of
> QEMU HBA hardware emulation.  However both of these currently require
> interaction with the QEMU block layer and extra overhead of OS
> independent code running in user-space connected to our existing
> SG_IO/BSG interfaces.  This also has the limitiation that we currently
> cannot perform explict SCSI Initiator Port access management for LUNs
> also present via TCM_Loop SCSI LLD driver on the KVM host.
> 
> Recently there has been an off-list discussion between myself and
> members of the QEMU/KVM community, and they have expressed an interest
> in seeing a native paravirtualized SCSI passthrough available to QEMU
> guests with direct interaction into TCM fabric module code running on
> KVM host.  This would be using existing CDB level port emulation code in
> target_core_fabric_lib.c, and (mostly) generic fabric control plane in
> target_core_fabric_configfs.c..  Beyond the TCM fabric module specifics,
> this will require:
> 
> *) An asynchronous I/O capable virtio interface for KVM that does not
> require direct QEMU block-layer interaction
> 
> *) A multi-fabric WWN naming capable TCM fabric module using native
> virtio-scsi connections to individual KVM guests for kernel-level
> passthrough into TCM backend storage.  (eg: similar to TCM_Loop, but w/o
> the Linux/SCSI LLD frontend, and full explict Initiator port NodeACL +
> MappedLUN control abstraction)
> 
> *) Guest OS specific para-virtualized SCSI driver packages (similar to
> virtio-blk) performing SCSI I/O handoff into passthrough interface via
> SCSI I_T nexues, with proper T10 WWN Port and LUN naming provided from
> underlying TCM v4.x infrastructure  (eg: full compat w/ existing SPC-3
> capable cluster clients)
> 

I agree with the 2 above points on a better scsi-based Kernel-only
passthrough code in the KVM host. But the 3rd point, could we not used any
one of the few LLDs that already exist? I think there are already 3 of these,
no? including the one Microsoft submitted.

> --nab
> 

Thanks
Boaz