linux-kernel - Re: [RFC/RFT PATCH 04/19] x86: Make the 64-bit bzImage always physically relocatable

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Message-ID: <20260112104746.6521740c@pumpkin>
Date: Mon, 12 Jan 2026 10:47:46 +0000
From: David Laight <david.laight.linux@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
 x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
 <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
 <dave.hansen@...ux.intel.com>, Josh Poimboeuf <jpoimboe@...nel.org>, Peter
 Zijlstra <peterz@...radead.org>, Kees Cook <kees@...nel.org>, Uros Bizjak
 <ubizjak@...il.com>, Brian Gerst <brgerst@...il.com>,
 linux-hardening@...r.kernel.org
Subject: Re: [RFC/RFT PATCH 04/19] x86: Make the 64-bit bzImage always
 physically relocatable

On Sun, 11 Jan 2026 20:01:02 -0800
"H. Peter Anvin" <hpa@...or.com> wrote:

> On 2026-01-08 01:25, Ard Biesheuvel wrote:
> > On x86_64, the physical placement of the kernel is independent from its
> > mapping in the 'High Kernel Mapping' range. This means that even a
> > position dependent kernel built without boot-time relocation support can
> > run from any suitably aligned physical address, and there is no need to
> > make this behavior dependent on whether or not the kernel is virtually
> > relocatable.
> > 
> > On i386, the situation is different, given that the physical and virtual
> > load offsets must be equal, and so only a relocatable kernel can be
> > loaded at a physical address that deviates from its build-time default.
> > 
> > Clarify this in Kconfig and in the code, and advertise the 64-bit
> > bzImage as loadable at any physical offset regardless of whether
> > CONFIG_RELOCATABLE is set. In practice, this makes little difference,
> > given that it defaults to 'y' and is a prerequisite for EFI_STUB and
> > RANDOMIZE_BASE, but it will help with some future refactoring of the
> > relocation code.
> >   
> 
> I don't see any reason to support non-relocatable kernels anymore. In fact, in
> a patchset I am working on I have already removed it.

For just 64bit, or 32bit as well?
The 'bloat' for 32bit will be higher due to the lack of pc-relative
addressing.

	David

> 
> 	-hpa
> 
>