linux-kernel - Re: [PATCH 11/11] x86: drop 32-bit KVM host support

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Message-ID: <Z1B1phcpbiYWLgCD@google.com>
Date: Wed, 4 Dec 2024 07:30:58 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: 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>, 
	"H. Peter Anvin" <hpa@...or.com>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Andy Shevchenko <andy@...nel.org>, Matthew Wilcox <willy@...radead.org>, 
	Davide Ciminaghi <ciminaghi@...dd.com>, Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH 11/11] x86: drop 32-bit KVM host support

On Wed, Dec 04, 2024, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> There are very few 32-bit machines that support KVM, the main exceptions
> are the "Yonah" Generation Xeon-LV and Core Duo from 2006 and the Atom
> Z5xx "Silverthorne" from 2008 that were all release just before their
> 64-bit counterparts.
> 
> Using KVM as a host on a 64-bit CPU using a 32-bit kernel generally
> works fine, but is rather pointless since 64-bit kernels are much better
> supported and deal better with the memory requirements of VM guests.
> 
> Drop all the 32-bit-only portions and the "#ifdef CONFIG_X86_64" checks
> of the x86 KVM code and add a Kconfig dependency to only allow building
> this on 64-bit kernels.

While 32-bit KVM doesn't need to be a thing for x86 usage, Paolo expressed concerns
that dropping 32-bit support on x86 would cause general 32-bit KVM support to
bitrot horribly.  32-bit x86 doesn't get much testing, but I do at least boot VMs
with it on a semi-regular basis.  I don't think we can say the same for other
architectures with 32-bit variants.

PPC apparently still has 32-bit users[1][2], and 32-bit RISC-V is a thing, so I
think we're stuck with 32-bit x86 for the time being. :-(

[1] https://lore.kernel.org/all/87zg4aveow.fsf@mail.lhotse
[2] https://lore.kernel.org/all/fc43f9eb-a60f-5c4a-a694-83029234a9c4@xenosoft.de