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Wed, 4 Dec 2024 16:14:55 -0500 (EST) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Date: Wed, 04 Dec 2024 22:14:33 +0100 From: "Arnd Bergmann" To: "Andy Shevchenko" Cc: "H. Peter Anvin" , "Brian Gerst" , "Arnd Bergmann" , linux-kernel@vger.kernel.org, x86@kernel.org, "Thomas Gleixner" , "Ingo Molnar" , "Borislav Petkov" , "Dave Hansen" , "Linus Torvalds" , "Andy Shevchenko" , "Matthew Wilcox" , "Sean Christopherson" , "Davide Ciminaghi" , "Paolo Bonzini" , kvm@vger.kernel.org Message-Id: In-Reply-To: References: <20241204103042.1904639-1-arnd@kernel.org> <20241204103042.1904639-6-arnd@kernel.org> <13308b89-53d1-4977-970f-81b34f40f070@app.fastmail.com> Subject: Re: [PATCH 05/11] x86: remove HIGHMEM64G support Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On Wed, Dec 4, 2024, at 19:37, Andy Shevchenko wrote: > On Wed, Dec 4, 2024 at 6:57=E2=80=AFPM Arnd Bergmann w= rote: >> On Wed, Dec 4, 2024, at 17:37, H. Peter Anvin wrote: >> > On December 4, 2024 5:29:17 AM PST, Brian Gerst = wrote: >> >>> >> >>> Removing this also drops the need for PHYS_ADDR_T_64BIT and SWIOT= LB. >> >>> 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. > > Theoretically swiotlb can be useful on 32-bit machines as well for the > DMA controllers that have < 32-bit mask. Dunno if swiotlb was designed > to run on 32-bit machines at all. Right, that is a possibility. However those machines would currently be broken on kernels without X86_PAE, since they don't select swiotlb. If anyone does rely on the current behavior of X86_PAE to support broken DMA devices, it's probably best to fix it in a different way. Arnd