linux-kernel - Re: [PATCH 05/11] x86: remove HIGHMEM64G support

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Message-Id: <13308b89-53d1-4977-970f-81b34f40f070@app.fastmail.com>
Date: Wed, 04 Dec 2024 17:55:34 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "H. Peter Anvin" <hpa@...or.com>, "Brian Gerst" <brgerst@...il.com>,
 "Arnd Bergmann" <arnd@...nel.org>
Cc: 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>,
 "Linus Torvalds" <torvalds@...ux-foundation.org>,
 "Andy Shevchenko" <andy@...nel.org>, "Matthew Wilcox" <willy@...radead.org>,
 "Sean Christopherson" <seanjc@...gle.com>,
 "Davide Ciminaghi" <ciminaghi@...dd.com>,
 "Paolo Bonzini" <pbonzini@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH 05/11] x86: remove HIGHMEM64G support

On Wed, Dec 4, 2024, at 17:37, H. Peter Anvin wrote:
> On December 4, 2024 5:29:17 AM PST, Brian Gerst <brgerst@...il.com> wrote:
>>>
>>> Removing this also drops the need for PHYS_ADDR_T_64BIT and SWIOTLB.
>>> PAE mode is still required to get access to the 'NX' bit on Atom
>>> 'Pentium M' and 'Core Duo' CPUs.
>
> By the way, there are 64-bit machines which require swiotlb.

What I meant to write here was that CONFIG_X86_PAE no longer
needs to select PHYS_ADDR_T_64BIT and SWIOTLB. I ended up
splitting that change out to patch 06/11 with a better explanation,
so the sentence above is just wrong now and I've removed it
in my local copy now.

Obviously 64-bit kernels still generally need swiotlb.

       Arnd