linux-kernel - Re: virtio-scsi spec (was Re: [PATCH] Add virtio-scsi to the virtio spec)

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Date:	Wed, 30 Nov 2011 17:36:52 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Hannes Reinecke <hare@...e.de>
CC:	Stefan Hajnoczi <stefanha@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...hat.com>,
	virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: virtio-scsi spec (was Re: [PATCH] Add virtio-scsi to the virtio
 spec)

On 11/30/2011 03:17 PM, Hannes Reinecke wrote:
>>   seg_max is the maximum number of segments that can be in a
>>     command. A bidirectional command can include seg_max input
>>     segments and seg_max output segments.
>>
> I would like to have the other request_queue limitations exposed
> here, too.
> Most notably we're missing the maximum size of an individual segment
> and the maximum size of the overall I/O request.

The virtio transport does not put any limit, as far as I know.

> As this is the host specification I really would like to see an host
> identifier somewhere in there.
> Otherwise we won't be able to reliably identify a virtio SCSI host.

I thought about it, but I couldn't figure out exactly how to use it. If 
it's just allocating 64 bits in the configuration space (with the 
stipulation that they could be zero), let's do it now. Otherwise a 
controlq command is indeed better, and it can come later.

But even if it's just a 64-bit value, then: 1) where would you place it 
in sysfs for userspace?  I can make up a random name, but existing user 
tools won't find it and that's against the design of virtio-scsi.  2) 
How would it be encoded as a transport ID?  Is it FC, or firewire, or 
SAS, or what?

> Plus you can't calculate the ITL nexus information, making
> Persistent Reservations impossible.

They are not impossible, only some features such as SPEC_I_PT.  If you 
use NPIV or iSCSI in the host, then the persistent reservations will 
already get the correct initiator port.  If not, much more work is needed.

> We should be adding
>
> VIRTIO_SCSI_S_BUSY
>
> for a temporary failure, indicating that a command retry
> might be sufficient to clear this situation.
> Equivalent to VIRTIO_SCSI_S_NEXUS_FAILURE, but issuing a retry on
> the same path.

... and equivalent to DID_BUS_BUSY.  Assuming no other major objections, 
I will add and resubmit in a few days.

Paolo

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