Re: eliminating 166G limit (was Re: [Xen-devel] Problem with nr_nodes on large memory NUMA machine)

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> 27.11.07 10:21 >>>
>On 27/11/07 09:00, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:
>
>>> I don't get how your netback approach works. The pages we transfer do not
>>> originate from netback, so it has little control over them. And, even if it
>>> did, when we allocate pages for network receive we do not know which
>>> domain's packet will end up in each buffer.
>> 
>> Oh, right, I mixed up old_mfn and new_mfn in netbk_gop_frag(). Nevertheless
>> netback could take care of this by doing the copying there, as at that point 
>> i
>> already knows the destination domain.
>
>You may not know constraints on that domain's max_mfn though. We could add
>an interface to Xen to interrogate that, but generally it's not something we
>probably want to expose outside of Xen and the guest itself.

What constraints other than the guest's address size influence its max_mfn?
Of course, if there's anything beyond the address size, then having a way to
obtain the constraint explicitly would be desirable. But otherwise (and as
fallback) using 37 bits (128G) seems quite reasonable.

>>> Personally I think doing it in Xen is perfectly good enough for supporting
>>> this very out-of-date network receive mechanism.
>> 
>> I'm not just concerned about netback here. The interface exists, and other
>> users might show up and/or exist already. Whether it would be acceptable
>> for them to do allocation and copying is unknown. You'd therefore either
>> need a way to prevent future users of the transfer mechanism, or set proper
>> requirements on its use. I think that placing extra requirements on the user
>> of the interface is better than introducing extra (possibly hard to 
>> reproduce/
>> recognize/debug) possibilities of failure.
>
>The interface is obsolete.

Then it should be clearly indicated as such, e.g. by a mechanism similar to
deprecated_irq_flag() in Linux 2.6.22. And as a result, its use in netback 
should
then probably be conditional upon an extra config option, which could at once
be used to provide a note to Xen that the feature isn't being used so that the
function could return -ENOSYS and the clipping could be avoided/reverted.

Jan


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