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Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX possible entries

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On 15/11/16 01:11, Alex Thorlton wrote:
> Hey everyone,
> 
> We're having problems with large systems hitting a BUG in
> xen_memory_setup, due to extra e820 entries created in the
> XENMEM_machine_memory_map callback.  The change in the patch gets things
> working, but Boris and I wanted to get opinions on whether or not this
> is the appropriate/entire solution, which is why I've sent it as an RFC
> for now.
> 
> Boris pointed out to me that E820_X_MAX is only large when CONFIG_EFI=y,
> which is a detail worth discussig.  He proposed possibly adding
> CONFIG_XEN to the conditions under which we set E820_X_MAX to a larger
> value than E820MAX, since the Xen e820 table isn't bound by the
> zero-page memory limitations.
> 
> I do *slightly* question the use of E820_X_MAX here, only from a
> cosmetic prospective, as I believe this macro is intended to describe
> the maximum size of the extended e820 table, which, AFAIK, is not used
> by the Xen HV.  That being said, there isn't exactly a "more
> appropriate" macro/variable to use, so this may not really be an issue.
> 
> Any input on the patch, or the questions I've raised above is greatly
> appreciated!

While I think extending the e820 table is the right thing to do I'm
questioning the assumptions here.

Looking briefly through the Xen hypervisor sources I think it isn't
yet ready for such large machines: the hypervisor's e820 map seems to
be still limited to 128 e820 entries. Jan, did I overlook an EFI
specific path extending this limitation?

In case I'm right the Xen hypervisor should be prepared for a larger
e820 map, but this won't help alone as there would still be additional
entries for the IOAPICs created.

So I think we need something like:

#define E820_XEN_MAX (E820_X_MAX + MAX_IO_APICS)

and use this for sizing xen_e820_map[].


Juergen

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[RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX possible entries Alex Thorlton <athorlton@sgi.com> - 2016-11-15 01:20 +0100
  Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX possible  entries Juergen Gross <jgross@suse.com> - 2016-11-15 07:40 +0100
    Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX  possible entries "Jan Beulich" <JBeulich@suse.com> - 2016-11-15 08:20 +0100
    Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX possible  entries Juergen Gross <jgross@suse.com> - 2016-11-15 08:40 +0100
      Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX  possible entries "Jan Beulich" <JBeulich@suse.com> - 2016-11-15 09:10 +0100
      Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX possible  entries Juergen Gross <jgross@suse.com> - 2016-11-15 09:50 +0100
        Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX  possible entries "Jan Beulich" <JBeulich@suse.com> - 2016-11-15 10:50 +0100
        Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX possible  entries Juergen Gross <jgross@suse.com> - 2016-11-15 11:00 +0100
          Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX  possible entries "Jan Beulich" <JBeulich@suse.com> - 2016-11-15 11:50 +0100
          Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX possible  entries Juergen Gross <jgross@suse.com> - 2016-11-15 12:10 +0100
            Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX  possible entries "Jan Beulich" <JBeulich@suse.com> - 2016-11-15 12:20 +0100
          Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX  possible entries Alex Thorlton <athorlton@sgi.com> - 2016-11-15 16:30 +0100
            Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX possible  entries Juergen Gross <jgross@suse.com> - 2016-11-16 07:10 +0100

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