linux-kernel - Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X

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Date:   Tue, 15 Nov 2016 07:33:26 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Alex Thorlton <athorlton@....com>, linux-kernel@...r.kernel.org,
        Jan Beulich <JBeulich@...e.com>
Cc:     Russ Anderson <rja@....com>,
        David Vrabel <david.vrabel@...rix.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [RFC PATCH] xen/x86: Increase xen_e820_map to E820_X_MAX possible
 entries

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