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Re: [Xen-devel] [PATCH 13/13] xen: allow more than 512 GB of RAM for 64 bit pv-domains

On 02/19/2015 06:51 PM, David Vrabel wrote:
On 18/02/2015 11:51, Juergen Gross wrote:
On 02/18/2015 12:18 PM, David Vrabel wrote:
On 18/02/15 06:52, Juergen Gross wrote:

+if X86_64
+    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
+      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"

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.

How is the toolstack supposed to indicate this?

I don't know.  A bit in the shared info perhaps?

Okay, I'd be fine with this. CC'ed the hypervisor maintainers to enable
them to object early. :-)

I'd be more than happy to get rid of that option. For Dom0 you seem to
have changed your mind (you rejected enabling >512GB as default last

Since for dom0 it's only crash analysis that might be impacted, removing
the option is fine.  Anyone using such a tool probably know what they're

Fine, I'll remove it.

Doing some more tests I found the command line option is problematic:
The option seems to be evaluated only after it is needed (I did the
first tests using the config option). Can we get rid of the option
even for domU? Or do I have to pre-scan the command line for the option?

I don't what people to be able to inadvertantly create domUs that cannot
be saved/migrated.  Can you think of another mechanism?

No, I hoped you'd have an idea.

I'll try to parse the command line options a little bit earlier.


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

What about Dom0?

This bit is for dom0 and domU.

Okay, but which toolstack would set the indicator for dom0? The
hypervisor? Would it set this bit in any case, or depending on a
command line option?


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