linux-kernel - Re: [PATCH 5/5] xen: tolerate ACPI NVS memory overlapping with Xen allocated memory

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Message-ID: <90d67e10-6e35-487e-a9e7-611a0fa3b00b@suse.com>
Date: Wed, 7 Aug 2024 14:05:33 +0200
From: Jan Beulich <jbeulich@...e.com>
To: Juergen Gross <jgross@...e.com>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
 Marek Marczykowski-Górecki
 <marmarek@...isiblethingslab.com>, linux-kernel@...r.kernel.org,
 x86@...nel.org
Subject: Re: [PATCH 5/5] xen: tolerate ACPI NVS memory overlapping with Xen
 allocated memory

On 07.08.2024 12:41, Juergen Gross wrote:
> In order to minimize required special handling for running as Xen PV
> dom0, the memory layout is modified to match that of the host. This
> requires to have only RAM at the locations where Xen allocated memory
> is living. Unfortunately there seem to be some machines, where ACPI
> NVS is located at 64 MB, resulting in a conflict with the loaded
> kernel or the initial page tables built by Xen.
> 
> As ACPI NVS needs to be accessed by the kernel only for saving and
> restoring it across suspend operations, it can be relocated in the
> dom0's memory map by swapping it with unused RAM (this is possible
> via modification of the dom0 P2M map).

While the kernel may not (directly) need to access it for other purposes,
what about AML accessing it? As you can't advertise the movement to ACPI,
and as non-RAM mappings are carried out by drivers/acpi/osl.c:acpi_map()
using acpi_os_ioremap(), phys-to-machine translations won't cover for
that (unless I'm overlooking something, which unfortunately seems like I
might be).

Even without that I wonder in how far tools (kexec?) may be misguided by
relocating non-RAM memory ranges. Even more so when stuff traditionally
living below the 4G boundary suddenly moves far beyond that.

Jan