linux-kernel - Re: [Xen-devel] [PATCH 3/3] xen: eliminate scalability issues from initial mapping setup

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Date:	Thu, 04 Sep 2014 15:31:44 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andrew Cooper" <andrew.cooper3@...rix.com>
Cc:	"David Vrabel" <david.vrabel@...rix.com>,
	<xen-devel@...ts.xensource.com>, <boris.ostrovsky@...cle.com>,
	<konrad.wilk@...cle.com>, "Juergen Gross" <JGross@...e.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 3/3] xen: eliminate scalability issues
 from initial mapping setup

>>> On 04.09.14 at 15:02, <andrew.cooper3@...rix.com> wrote:
> On 04/09/14 13:59, David Vrabel wrote:
>> On 04/09/14 13:38, Juergen Gross wrote:
>>> Direct Xen to place the initial P->M table outside of the initial
>>> mapping, as otherwise the 1G (implementation) / 2G (theoretical)
>>> restriction on the size of the initial mapping limits the amount
>>> of memory a domain can be handed initially.
>> The three level p2m limits memory to 512 GiB on x86-64 but this patch
>> doesn't seem to address this limit and thus seems a bit useless to me.
> 
> Any increase of the p2m beyond 3 levels will need to come with
> substantial libxc changes first.  3 level p2ms are hard coded throughout
> all the PV build and migrate code.

No, there no such dependency - the kernel could use 4 levels at
any time (sacrificing being able to get migrated), making sure it
only exposes the 3 levels hanging off the fourth level (or not
exposing this information at all) to external entities making this
wrong assumption.

Jan

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