Re: [Qemu-devel] [RFC PATCH] implement vmware pvscsi device

[Hannes Reinecke <hare@suse.de>]

On 04/15/2011 04:17 PM, Paolo Bonzini wrote:
> On 04/15/2011 04:01 PM, Stefan Hajnoczi wrote:
>> I think SCSI brings many benefits.  Guests can deal with it better
>> than these alien vdX virtio-blk devices, which makes migration easier.
>> It becomes possible to attach many disks without burning through free
>> PCI slots.  We don't need to update guests to add cache control,
>> discard, and other commands because they are part of SCSI.  We can
>> pass through more exotic devices.  The list goes on...
> 
> And we also have to reimplement all of MMC. :)
> 
> A few questions:
> 
> 1) Do you have anything posted for the virtio-scsi spec?  I had started
> working on one, but I haven't yet made it final.  It included also
> hotplug/unplug.  I can send it out on Monday.
> 
> 2) Have you thought about making scsi-disk and scsi-generic provide a
> logical unit rather than a target?  Otherwise passthrough of a whole
> host or target becomes hard or messy.
> 
> 3) Since I noticed Hannes is CCed, my next step for vmw_pvscsi would be
> to dust off his patches to remove the bounce buffers, and see how they
> apply to vmw_pvscsi.  But I'd like to avoid duplicated work if possible.
> 

Argl.

Why vmw_pvscsi? Any paravirtualized driver doesn't improve the situation
here; we still wouldn't have a driver for unmodified guests.
So either emulate existing drivers (like megasas :-) or go the full
route and do a proper virtio-scsi.

As for the bounce buffers thing:
Good luck. Paul Brook absolutely insists on having them, but they kill
performance for any sane backend. And both are basically impossible to
reconcile; tried it once but got pushed back.

And after about the third attempt I gave up. Let me know if you have
more luck here.

But keep me in the loop for the virtio-scsi spec. I do have some ideas
what needs to get in there.
As I think hch has.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke              zSeries & Storage
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