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

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Message-ID: <CAMzpN2gTUks3K3Hvwq3MEVBCN-9HHTLM4+FNdHkuQOmgX0Tfjg@mail.gmail.com>
Date: Wed, 4 Dec 2024 09:02:48 -0500
From: Brian Gerst <brgerst@...il.com>
To: 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>, "H. Peter Anvin" <hpa@...or.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 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