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Re: [PATCH V3 (resend) 13/19] x86/setup: Do not create valid mappings when directmap=no
- To: Elias El Yandouzi <eliasely@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 15 May 2024 17:59:21 +0200
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- Cc: julien@xxxxxxx, pdurrant@xxxxxxxxxx, dwmw@xxxxxxxxxx, Hongyan Xia <hongyxia@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Julien Grall <jgrall@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 15 May 2024 15:59:35 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 13.05.2024 15:40, Elias El Yandouzi wrote:
> From: Hongyan Xia <hongyxia@xxxxxxxxxx>
>
> Create empty mappings in the second e820 pass. Also, destroy existing
> direct map mappings created in the first pass.
>
> To make xenheap pages visible in guests, it is necessary to create empty
> L3 tables in the direct map even when directmap=no, since guest cr3s
> copy idle domain's L4 entries, which means they will share mappings in
> the direct map if we pre-populate idle domain's L4 entries and L3
> tables. A helper is introduced for this.
Hmm. On one hand this may reduce memory consumption some, when large
ranges of MFNs aren't allocated as Xen heap pages. Otoh this increases
memory needs when Xen heap pages actually need mapping. I wonder whether
the (presumably less intrusive) change of merely altering permissions
from PAGE_HYPERVISOR to _PAGE_NONE|MAP_SMALL_PAGES wouldn't be better.
> Also, after the direct map is actually gone, we need to stop updating
> the direct map in update_xen_mappings().
What is this about? You only alter setup.c here.
Jan
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