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

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Message-ID: <87le6ab2bn.fsf@email.froward.int.ebiederm.org>
Date: Fri, 22 Mar 2024 12:31:40 -0500
From: "Eric W. Biederman" <ebiederm@...ssion.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: 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>,  Simon Horman
 <horms@...ge.net.au>,  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.

Dave Hansen <dave.hansen@...el.com> writes:

> 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.

So this change is a partial revert of a change that broke kexec in
existing configurations.  To fix a regression that breaks kexec.

I don't have enough to know which systems broke.  So I don't know
the difficulty in fixing systems.

Dave do you know what synthetic entries need to be added to the
memory maps to fix the regression?  If not we should go with this
and if we care enough we can add the synthetic entries later.

Eric