RE: [PATCH 00/25] Staging: hv: Cleanup vmbus driver code

[KY Srinivasan]

> -----Original Message-----
> From: Christoph Hellwig [mailto:hch@xxxxxxxxxxxxx]
> Sent: Wednesday, April 27, 2011 8:19 AM
> To: KY Srinivasan
> Cc: Christoph Hellwig; Greg KH; gregkh@xxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> devel@xxxxxxxxxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxx
> Subject: Re: [PATCH 00/25] Staging: hv: Cleanup vmbus driver code
> 
> On Wed, Apr 27, 2011 at 11:47:03AM +0000, KY Srinivasan wrote:
> > On the host side, Windows emulates the  standard PC hardware
> > to permit hosting of fully virtualized operating systems.
> > To enhance disk I/O performance, we support a virtual block driver.
> > This block driver currently handles disks that have been setup as IDE
> > disks for the guest - as specified in the guest configuration.
> >
> > On the SCSI side, we emulate a SCSI HBA. Devices configured
> > under the SCSI controller for the guest are handled via this
> > emulated HBA (SCSI front-end). So, SCSI disks configured for
> > the guest are handled through native SCSI upper-level drivers.
> > If this SCSI front-end driver is not loaded, currently, the guest
> > cannot see devices that have been configured as SCSI devices.
> > So, while the virtual block driver described earlier could potentially
> > handle all block devices, the implementation choices made on the host
> > will not permit it. Also, the only SCSI device that can be currently configured
> > for the guest is a disk device.
> >
> > Both the block device driver (hv_blkvsc) and the SCSI front-end
> > driver (hv_storvsc) communicate with the host via unique channels
> > that are implemented as bi-directional ring buffers. Each (storage)
> > channel carries with it enough state to uniquely identify the device on
> > the host side. Microsoft has chosen to use SCSI verbs for this storage channel
> > communication.
> 
> This doesn't really explain much at all.  The only important piece
> of information I can read from this statement is that both blkvsc
> and storvsc only support disks, but not any other kind of device,
> and that chosing either one is an arbitrary seletin when setting up
> a VM configuration.
> 
> But this still isn't an excuse to implement a block layer driver for
> a SCSI protocol, and it doesn't not explain in what way the two
> protocols actually differ.  You really should implement blksvs as a SCSI
> LLDD, too - and from the looks of it it doesn't even have to be a
> separate one, but just adding the ids to storvsc would do the work.

On the host-side, as part of configuring a guest  you can specify block devices
as being under an IDE controller or under a
SCSI controller. Those are the only options you have. Devices configured under
the IDE controller cannot be seen in the guest under the emulated SCSI front-end which is
the scsi driver (storvsc_drv). So, when you do a bus scan in the emulated scsi front-end,
the devices enumerated will not include block devices configured under the IDE 
controller. So, it is not clear to me how I can do what you are proposing given the 
restrictions imposed by the host.

Regards,

K. Y
 

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