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[Xen-devel] RE: Tmem vs order>0 allocation, workaround RFC

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: [Xen-devel] RE: Tmem vs order>0 allocation, workaround RFC
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Tue, 16 Feb 2010 07:31:58 -0800 (PST)
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, kurt.hackel@xxxxxxxxxx, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, TimDeegan <Tim.Deegan@xxxxxxxxxxxxx>, Patrick Colp <pjcolp@xxxxxxxxx>, Grzegorz Milos <gm281@xxxxxxxxx>, Keir, Fraser <keir.fraser@xxxxxxxxxxxxx>, AndrewPeace <Andrew.Peace@xxxxxxxxxxxxx>
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> From: Jan Beulich [mailto:JBeulich@xxxxxxxxxx]
> Subject: RE: Tmem vs order>0 allocation, workaround RFC
> 
> >>> Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> 16.02.10 16:05 >>>
> >Under what circumstances does dom0 require single-page-below-4G
> >allocations?  Is it only for bounce buffers for PCI passthrough
> >of old devices with 32-bit addressing limitations?  Or am I
> >missing a much more common case?
> 
> Not just for pass-through; all devices only supporting 32-bit
> addressing would have such requirements, and particularly common
> ones are display adapters which have DRM/AGP drivers loaded for
> them.

Right, but those are statically allocated when dom0 is
launched, not dynamically allocated later after tmem
(or other memory allocation technologies) begin working,
right?  Whereas pass-through devices would need below-4G
pages later?

(And 32-bit devices in a 1TB machine seems a bit of a
stretch, but I suppose it is good to enumerate all the
cases.)

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