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

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Message-ID: <64221AAE-4A99-42D8-A78F-9B1B866CB24A@zytor.com>
Date: Wed, 04 Dec 2024 07:53:47 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Arnd Bergmann <arnd@...db.de>, 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 December 4, 2024 5:43:28 AM PST, Arnd Bergmann <arnd@...db.de> wrote:
>On Wed, Dec 4, 2024, at 14:29, Brian Gerst wrote:
>> On Wed, Dec 4, 2024 at 5:34 AM Arnd Bergmann <arnd@...nel.org> wrote:
>>>
>>>  - 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.
>
>Can you give some more background on this?
>
>It's clear that one can run a virtual machine this way and it
>currently works, but are you able to construct a case where this
>is a good idea, compared to running the same userspace with a
>64-bit kernel?
>
>>From what I can tell, any practical workload that requires
>8GB of total RAM will likely run into either the lowmem
>limits or into virtual addressig limits, in addition to the
>problems of 32-bit kernels being generally worse than 64-bit
>ones in terms of performance, features and testing.
>
>      Arnd
>

The biggest proven is that without HIGHMEM you put a limit of just under 1 GB (892 MiB if I recall correctly), *not* 4 GB, on 32-bit kernels. That is *well* below the amount of RAM present in late-era 32-bit legacy systems, which were put in production as "recently" as 20 years ago and may still be in niche production uses. Embedded systems may be significantly more recent; I know for a fact that 32-bit systems were put in production in the 2010s.