linux-kernel - Re: [PATCH V3] xen: eliminate scalability issues from initial mapping setup

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Date:	Wed, 24 Sep 2014 16:03:45 +0200
From:	Juergen Gross <jgross@...e.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com,
	jbeulich@...e.com
Subject: Re: [PATCH V3] xen: eliminate scalability issues from initial mapping
 setup

On 09/24/2014 03:20 PM, David Vrabel wrote:
> On 17/09/14 15:59, 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.
>>
>> As the initial P->M table is copied rather early during boot to
>> domain private memory and it's initial virtual mapping is dropped,
>> the easiest way to avoid virtual address conflicts with other
>> addresses in the kernel is to use a user address area for the
>> virtual address of the initial P->M table. This allows us to just
>> throw away the page tables of the initial mapping after the copy
>> without having to care about address invalidation.
>>
>> It should be noted that this patch won't enable a pv-domain to USE
>> more than 512 GB of RAM. It just enables it to be started with a
>> P->M table covering more memory. This is especially important for
>> being able to boot a Dom0 on a system with more than 512 GB memory.
>
> This doesn't seem to work.  It crashes when attempting to construct
> the page tables.  Have these patches been tested on a host with > 512 GiB?

Not yet. I did a code review and was pretty sure the memory above 512GB
would be ignored - seems as if I was wrong.

I'll have access to a machine with 1TB RAM soon, so I'll try to test a
patch which really does what I thought should be done: ignoring the
memory above 512GB.

Thanks for testing!


Juergen
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