From mboxrd@z Thu Jan  1 00:00:00 1970
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Subject: Re: [PATCH] xen/domctl: lower loglevel of
 XEN_DOMCTL_memory_mapping
Date: Thu, 10 Sep 2015 20:59:20 -0400
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To: "Chen, Tiejun" <tiejun.chen@intel.com>
Cc: Keir Fraser <keir@xen.org>, Ian Campbell <ian.campbell@citrix.com>, Tim Deegan <tim@xen.org>, IanJackson <ian.jackson@eu.citrix.com>, xen-devel@lists.xen.org, Jan Beulich <JBeulich@suse.com>, Malcolm Crossley <malcolm.crossley@citrix.com>
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On Fri, Sep 11, 2015 at 08:44:50AM +0800, Chen, Tiejun wrote:
> >>Right, that's one of the things that would need taking care of.
> >>(Whether enforcing an upper limit is actually needed I'm not
> >>sure - we generally allow the admin to shoot himself in the foot
> >>if he wants to. And whether the lower limit should be 64 instead
> >>of just ensuring the limit is not zero is another question.)
> >
> >64 was semi-arbitrary  - it ended up giving good latency on
> >highly scalar machines (8 socket). Higher numbers ended up
> >affecting the latency.
> >
> >But higher numbers on small socket machines were OK.
> >(As they do not have 8 IOMMU VT-d chipsets all potentially
> >flodding the QPI with serialized cache flushes).
> >>
> 
> So we should make this range [8, <Its up to the user>] here, but 64 by
> default. Right?

Not sure I follow you. The 8 was based on 8 socket machines having 8
IOMMUs..

If you want a formula I would do:

#define MAX_SOCKETS 8

 max_pfns = pow(2,(MAX_SOCKETS - (max(nr_iommus(), MAX_SOCKETS)))) * 64;

Where nr_iommus would have to be somehow implemented, ditto for pow.

This should give you:
 8	-> 64
 7	-> 128
 6	-> 256
 5	-> 512
 4	-> 1024
 3	-> 2048
 2	-> 4096
 1	-> 16384

> 
> Thanks
> Tiejun
>