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Re: [Xen-devel] [PATCH 3/6] x86/boot: Remove the preconstructed low 16M superpage mappings


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Wed, 8 Jan 2020 19:41:15 +0000
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  • Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Wed, 08 Jan 2020 19:41:30 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 08/01/2020 11:23, Jan Beulich wrote:
>>>  This would then also ease shrinking the build
>>> time mappings further, e.g. to the low 1Mb (instead of touching
>>> several of the places you touch now, it would again mainly be an
>>> adjustment to BOOTSTRAP_MAP_BASE, alongside the assembly file
>>> changes needed).
>> ... as you correctly identify here, it is a property of the prebuilt
>> tables (in l?_identmap[]), not a property of where we chose to put the
>> dynamic boot mappings (in the l?_bootmap[]).  Another change (blocked
>> behind the above bug) moves BOOTSTRAP_MAP_BASE to be 1G to reduce the
>> chance of an offset from a NULL pointer hitting a present mapping.
> Right, BOOTSTRAP_MAP_BASE was (ab)used for a 2nd purpose. But this
> would better be dealt with by introducing a new manifest constant
> (e.g. PREBUILT_MAP_LIMIT) instead of open-coding 2Mb everywhere.

I'm hoping to get rid of even this, (although it is complicated by
CONFIG_VIDEO's blind use of the legacy VGA range).

> Plus there's (aiui) a PREBUILT_MAP_LIMIT <= BOOTSTRAP_MAP_BASE
> requirement, which would better be verified (e.g. by a BUILD_BUG_ON())
> then.

Is there?  I don't see a real connection between the two, even in this
patch.

~Andrew

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