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

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Date:	Fri, 05 Sep 2014 09:55:47 +0200
From:	Juergen Gross <jgross@...e.com>
To:	Andrew Cooper <andrew.cooper3@...rix.com>,
	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, linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH 3/3] xen: eliminate scalability issues from
 initial mapping setup

On 09/04/2014 04:43 PM, Andrew Cooper wrote:
> 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.

I always thought the 3 level p2m is constructed by the kernel, not by
the tools.

It starts with the linear p2m list anchored at xen_start_info->mfn_list,
constructs the p2m tree and writes the p2m_top_mfn mfn to
HYPERVISOR_shared_info->arch.pfn_to_mfn_frame_list_list

See comment in the kernel source arch/x86/xen/p2m.c

So booting with a larger p2m list can be handled completely by the
kernel itself.

>
> 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".

I'd rather fix it in a clean way.

I think the best way to do it would be an indicator in the p2m array
anchor, e.g. setting 1<<61 in pfn_to_mfn_frame_list_list. This will
result in an early error with old tools:
"Couldn't map p2m_frame_list_list"


Juergen
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