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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <dccca669-7981-4f36-a701-0692c6b5c958@app.fastmail.com>
Date: Wed, 04 Dec 2024 22:14:33 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Andy Shevchenko" <andy.shevchenko@...il.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, "Brian Gerst" <brgerst@...il.com>,
 "Arnd Bergmann" <arnd@...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>,
 "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 19:37, Andy Shevchenko wrote:
> On Wed, Dec 4, 2024 at 6:57 PM Arnd Bergmann <arnd@...db.de> wrote:
>> 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.
>
> 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