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

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Date:	Thu, 4 Sep 2014 15:43:45 +0100
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Jan Beulich <JBeulich@...e.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 15:31, Jan Beulich wrote:
>>>> 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
>

That would require that the PV kernel must start with a 3 level p2m and
fudge things afterwards.

At a minimum, I would expect a patch to libxc to detect a 4 level PV
guest and fail with a meaningful error, rather than an obscure "m2p
doesn't match p2m for mfn/pfn X".

~Andrew
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