Re: [RFC 0/5] KVM: drop 32-bit host support on all architectures

From: Paolo Bonzini <pbonzini@redhat.com>
To: Arnd Bergmann <arnd@arndb.de>,
	"A. Wilcox" <AWilcox@wilcox-tech.com>,
	Arnd Bergmann <arnd@kernel.org>
Cc: kvm@vger.kernel.org,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Huacai Chen <chenhuacai@kernel.org>,
	Jiaxun Yang <jiaxun.yang@flygoat.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Naveen N Rao <naveen@kernel.org>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	Alexander Graf <graf@amazon.com>,
	Crystal Wood <crwood@redhat.com>,
	Anup Patel <anup@brainfault.org>,
	Atish Patra <atishp@atishpatra.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Sean Christopherson <seanjc@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	David Woodhouse <dwmw2@infradead.org>,
	Paul Durrant <paul@xen.org>, Marc Zyngier <maz@kernel.org>,
	linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org,
	linux-riscv@lists.infradead.org
Subject: Re: [RFC 0/5] KVM: drop 32-bit host support on all architectures
Date: Fri, 13 Dec 2024 09:20:05 +0100	[thread overview]
Message-ID: <79b9abfe-cfb8-4ef0-8a4b-7b87787e6549@redhat.com> (raw)
In-Reply-To: <6e971322-8b21-4d73-922c-a6032c6fe9bd@app.fastmail.com>

On 12/13/24 09:03, Arnd Bergmann wrote:
> On Fri, Dec 13, 2024, at 04:51, A. Wilcox wrote:
>> On Dec 12, 2024, at 6:55 AM, Arnd Bergmann <arnd@kernel.org> wrote:
>>> From: Arnd Bergmann <arnd@arndb.de>
>>>
>>> I submitted a patch to remove KVM support for x86-32 hosts earlier
>>> this month, but there were still concerns that this might be useful for
>>> testing 32-bit host in general, as that remains supported on three other
>>> architectures. I have gone through those three now and prepared similar
>>> patches, as all of them seem to be equally obsolete.
>>>
>>> Support for 32-bit KVM host on Arm hardware was dropped back in 2020
>>> because of lack of users, despite Cortex-A7/A15/A17 based SoCs being
>>> much more widely deployed than the other virtualization capable 32-bit
>>> CPUs (Intel Core Duo/Silverthorne, PowerPC e300/e500/e600, MIPS P5600)
>>> combined.
>>
>>
>> I do use 32-bit KVM on a Core Duo “Yonah” and a Power Mac G4 (MDD), for
>> purposes of bisecting kernel issues without having to reboot the host
>> machine (when it can be duplicated in a KVM environment).
>>
>> I suppose it would still be possible to run the hosts on 6.12 LTS for
>> some time with newer guests, but it would be unfortunate.
> 
> Would it be an option for you to just test those kernels on 64-bit
> machines? I assume you prefer to do native builds on 32-bit hardware
> because that fits your workflow, but once you get into debugging
> in a virtual machine, the results should generally be the same when
> building and running on a 64-bit host for both x86-32 and ppc32-classic,
> right?

Certainly for x86-32; ppc32 should be able to use PR-state (aka trap and 
emulate) KVM on a 64-bit host but it's a bit more picky.  Another 
possibility for ppc32 is just emulation with QEMU.

Paolo

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