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

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Message-ID: <ed6a32ef-85f0-4f90-883b-911ef0964984@intel.com>
Date: Mon, 1 Apr 2024 11:03:20 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Steve Wahl <steve.wahl@....com>, "Eric W. Biederman"
 <ebiederm@...ssion.com>
Cc: Russ Anderson <rja@....com>, Ingo Molnar <mingo@...nel.org>,
 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>,
 Dimitri Sivanich <sivanich@....com>
Subject: Re: [PATCH] x86/mm/ident_map: Use full gbpages in identity maps
 except on UV platform.

On 4/1/24 08:15, Steve Wahl wrote:
> From that point of view, it does make sense to special case UV systems
> for this.  The restricted areas we're talking about are not in the map
> when the bootloader is started on the UV platform.

Just to be clear what I'm looking for here: Special casing UV systems is
theoretically OK.  What I don't like is doing that in using GB pages or not.

It would be much nicer to have specific, precise information about what
UV needs done.  For instance, do we know where the special address range
is?  Is it fixed?  If so, I'd much rather have code that says: "Whoa,
don't map this range with *any* identity map page tables" versus
something targeted specifically at gbpages.