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

From: Juergen Gross
Date: Wed Sep 24 2014 - 10:03:52 EST

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!

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