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

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Message-ID: <FEB8B811-EA8F-41B4-B423-DBDE85AFA936@zytor.com>
Date: Wed, 04 Dec 2024 07:58:41 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Brian Gerst <brgerst@...il.com>, Arnd Bergmann <arnd@...db.de>
CC: 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 December 4, 2024 6:02:48 AM PST, Brian Gerst <brgerst@...il.com> wrote:
>On Wed, Dec 4, 2024 at 8:43 AM 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.
>
>I use a 32-bit VM to test 32-bit kernel builds.  I haven't benchmarked
>kernel builds with 4GB/8GB yet, but logically more memory would be
>better for caching files.
>
>
>Brian Gerst
>

For the record, back when kernel.org was still a 32-bit machine which, once would have thought, would have been ideal for caching files, rarely achieved more than 50% memory usage with which I believe was 8 GB RAM. The next generation was 16 GB x86-64.