linux-kernel - Re: [PATCH] x86/mm/ident_map: Use full gbpages in identity maps except on UV platform.

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Date: Fri, 22 Mar 2024 09:27:54 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Steve Wahl <steve.wahl@....com>, Dave Hansen
 <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 linux-kernel@...r.kernel.org,
 Linux regressions mailing list <regressions@...ts.linux.dev>,
 Pavin Joseph <me@...injoseph.com>, stable@...r.kernel.org,
 Eric Hagberg <ehagberg@...il.com>
Cc: Simon Horman <horms@...ge.net.au>, Eric Biederman
 <ebiederm@...ssion.com>, Dave Young <dyoung@...hat.com>,
 Sarah Brofeldt <srhb@....dk>, Russ Anderson <rja@....com>,
 Dimitri Sivanich <sivanich@....com>
Subject: Re: [PATCH] x86/mm/ident_map: Use full gbpages in identity maps
 except on UV platform.

On 3/22/24 09:21, Steve Wahl wrote:
> Some systems have ACPI tables that don't include everything that needs
> to be mapped for a successful kexec.  These systems rely on identity
> maps that include the full gigabyte surrounding any smaller region
> requested for kexec success.  Without this, they fail to kexec and end
> up doing a full firmware reboot.
> 
> So, reduce the use of GB pages only on systems where this is known to
> be necessary (specifically, UV systems).

Isn't this called "buggy firmware"?

I'd much rather add synthetic entries to the memory maps that have this
information than hack around it by assuming that things are within a
gigabyte.