Re: [Xen-devel] [PATCH] xen/domctl: lower loglevel of XEN_DOMCTL_memory_mapping

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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
> 

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