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

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Message-ID: <A0F192E7-EFD2-4DD4-8E84-764BF7210C6A@zytor.com>
Date: Wed, 04 Dec 2024 08:37:22 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Brian Gerst <brgerst@...il.com>, Arnd Bergmann <arnd@...nel.org>
CC: linux-kernel@...r.kernel.org, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>, 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 December 4, 2024 5:29:17 AM PST, Brian Gerst <brgerst@...il.com> wrote:
>On Wed, Dec 4, 2024 at 5:34 AM Arnd Bergmann <arnd@...nel.org> wrote:
>>
>> From: Arnd Bergmann <arnd@...db.de>
>>
>> The HIGHMEM64G support was added in linux-2.3.25 to support (then)
>> high-end Pentium Pro and Pentium III Xeon servers with more than 4GB of
>> addressing, NUMA and PCI-X slots started appearing.
>>
>> I have found no evidence of this ever being used in regular dual-socket
>> servers or consumer devices, all the users seem obsolete these days,
>> even by i386 standards:
>>
>>  - Support for NUMA servers (NUMA-Q, IBM x440, unisys) was already
>>    removed ten years ago.
>>
>>  - 4+ socket non-NUMA servers based on Intel 450GX/450NX, HP F8 and
>>    ServerWorks ServerSet/GrandChampion could theoretically still work
>>    with 8GB, but these were exceptionally rare even 20 years ago and
>>    would have usually been equipped with than the maximum amount of
>>    RAM.
>>
>>  - Some SKUs of the Celeron D from 2004 had 64-bit mode fused off but
>>    could still work in a Socket 775 mainboard designed for the later
>>    Core 2 Duo and 8GB. Apparently most BIOSes at the time only allowed
>>    64-bit CPUs.
>>
>>  - In the early days of x86-64 hardware, there was sometimes the need
>>    to run a 32-bit kernel to work around bugs in the hardware drivers,
>>    or in the syscall emulation for 32-bit userspace. This likely still
>>    works but there should never be a need for this any more.
>>
>> 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.
>
>8GB of memory is still useful for 32-bit guest VMs.
>
>
>Brian Gerst
>

By the way, there are 64-bit machines which require swiotlb.