Re: [Qemu-devel] [PATCH 01/58] spapr: proper qdevification

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/15/2011 05:14 AM, David Gibson wrote:
> Under PAPR, there is generally only
> supposed to be one SCSI target (disk / cd / whatever) per virtual scsi
> bus.  But the generic qdev code will, by default, keep assigning
> devices to the existing bus until it's full.  Any thoughts on how to
> sanely change that behaviour on a per-machine basis?

You could change the if_max_devs array in blockdev.c to something 
provided by the machines.

However, I'm not sure about this, for two reasons:

1) do you mean, in Linux terms, one target per SCSI _host_ or one target 
per SCSI _channel_?  i.e. if you looks at /sys/bus/scsi/devices, right 
now it looks like

    0:0:0:0    0:0:1:0     (two targets on the same host and channel)

Should it be?

    0:0:0:0    0:1:0:0     (one target per channel)

or

    0:0:0:0    1:0:0:0     (one target per host)

If it is the former, then you are simply hitting a limitation of the 
SCSI layer in QEMU and I do have patches to make assignment more 
flexible.  Based on the Linux VSCSI driver, and based on what SLOF does, 
I'd guess that's what you mean.

2) does this matter at all?  First, when doing "real world" 
virtualization you won't use the legacy options (neither -hda/-cdrom nor 
"-drive ...,if=scsi"), you would use -device to manually assign the 
devices to their buses.  Second, why should you care in the case of 
SCSI?  It seems like a very hard limitation to me, and unlike the PCI 
case it doesn't buy you anything in terms of isolation.

Paolo