Re: [Qemu-ppc] [PATCH] spapr: Reduce advertised max LUNs for spapr_vscsi

[Thomas Huth]

On 10/09/15 03:24, David Gibson wrote:
> On Wed, Sep 09, 2015 at 09:29:18AM +0200, Thomas Huth wrote:
>> On 09/09/15 09:19, David Gibson wrote:
>>> On Wed, Sep 09, 2015 at 08:25:34AM +0200, Thomas Huth wrote:
>>>> On 09/09/15 03:22, David Gibson wrote:
>>>>> The implementation of the PAPR paravirtual SCSI adapter currently
>>>>> allows up to 32 LUNs (max_lun == 31).  However the adapter isn't really
>>>>> designed to support lots of devices - the PowerVM implementation only
>>>>> ever puts one disk per vSCSI controller.
>>>>
>>>> Do you know how many LUNs are advertised by PowerVM?
>>>
>>> Well, what do you mean by "advertised".  AFAIK from the point of view
>>> of the guest, the number of LUNs is advertised per-target, not per
>>> controller.
>>
>> I mean, what's the highest LUN number that can be seen by a guest under
>> PowerVM? Is it always using only one LUN per controller, or is there a
>> way to change the amount of LUNs? (Sorry if I ask dumb questions ... I
>> do not have much experience with PowerVM yet)
> 
> Um.. I'm not sure, I have very little experience with PowerVM too.  I
> think with PowerVM it's usually real SCSI devices being passed
> through, rather than disk images, so presumably the SCSI target itself
> reports however many LUNs it has.  There may be a limitation in
> PowerVM, or in the AIX VIO server I think it typically backends onto,
> but I don't know what it is.
> 
> Since that limit has been in the guest side driver forever, presumbly
> no-one has hit LUNs > 8 in practice.
> 
>>>>> More specifically, the Linux guest side vscsi driver (the only one we
>>>>> really care about) is hardcoded to allow a maximum of 8 LUNs.
>>>>
>>>> So what about changing the vscsi driver in Linux instead to support more
>>>> LUNs?
>>>
>>> Doesn't help for existing guests.  Basically what I'm trying to
>>> achieve is for qemu to reject up-front configurations that are
>>> unlikely to actually work in the guest.
>>
>> I just wonder whether it makes sense to change the guest instead. In the
>> future, if we ever have guests that support more LUNs than 8 (maybe some
>> non-Linux guests like FreeBSD?), we've got to change QEMU back again...
>> OTOH, since this is just a one-line fix, it's likely ok to limit this to
>> 8 now - it's easy to revert if we ever need to, so I'm fine with that
>> change, I just wanted to discuss the other possibilites.
> 
> Remember that the spapr-vscsi device exists pretty much entirely to
> make transition simpler for existing PowerVM guests.  New guests
> (Linux or otherwise) intended to run under KVM should be using
> virtio-blk or virtio-scsi.

FWIW, I had a quick look at FreeBSD sources here:

https://svnweb.freebsd.org/base/stable/10/sys/powerpc/pseries/phyp_vscsi.c?revision=259204&view=markup

... and as far as I can see, they do not limit the LUNs to 8.
(I only spotted a "cpi->max_lun = ~(lun_id_t)(0);" in there).
So there indeed might also be older guests that support more than 8 LUNs.

 Thomas

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