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Date: Wed, 18 Feb 2015 11:18:55 +0000
From: David Vrabel <david.vrabel@citrix.com>
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To: Juergen Gross <jgross@suse.com>, <linux-kernel@vger.kernel.org>,
        <xen-devel@lists.xensource.com>, <konrad.wilk@oracle.com>,
        <david.vrabel@citrix.com>, <boris.ostrovsky@oracle.com>
Subject: Re: [Xen-devel] [PATCH 13/13] xen: allow more than 512 GB of RAM
 for 64 bit pv-domains
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On 18/02/15 06:52, Juergen Gross wrote:
> 
> +if X86_64
> +choice
> +	prompt "Support pv-domains larger than 512GB"
> +	default XEN_512GB_NONE
> +	help
> +	  Support paravirtualized domains with more than 512GB of RAM.
> +
> +	  The Xen tools and crash dump analysis tools might not support
> +	  pv-domains with more than 512 GB of RAM. This option controls the
> +	  default setting of the kernel to use only up to 512 GB or more.
> +	  It is always possible to change the default via specifying the
> +	  boot parameter "xen_512gb_limit".
> +
> +	config XEN_512GB_NONE
> +		bool "neither dom0 nor domUs can be larger than 512GB"
> +	config XEN_512GB_DOM0
> +		bool "dom0 can be larger than 512GB, domUs not"
> +	config XEN_512GB_DOMU
> +		bool "domUs can be larger than 512GB, dom0 not"
> +	config XEN_512GB_ALL
> +		bool "dom0 and domUs can be larger than 512GB"
> +endchoice
> +endif

This configuration option doesn't look useful to me.  Can we get rid of
it with runtime checking.  e.g.,

If dom0, enable >512G.
If domU, enable >512G if requested by command line option /or/ toolstack
indicates that it supports the linear p2m.

And

If max_pfn < 512G, populate 3-level p2m /unless/ toolstack indicates it
supports the linear p2m.

People using crash analysis tools that need the 3-level p2m can clamp
dom0 memory with the Xen command line option.  FWIW, the tool we use
doesn't need this.

David