linux-kernel - Re: [PATCH] KVM: x86: disable on 32-bit unless CONFIG

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Message-ID: <06d04f32-8403-4d7f-76a1-11a7fac3078e@redhat.com>
Date:   Fri, 17 Feb 2023 20:36:41 +0100
From:   Thomas Huth <thuth@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        "Daniel P. Berrange" <berrange@...hat.com>
Subject: Re: [PATCH] KVM: x86: disable on 32-bit unless CONFIG_BROKEN

On 29/09/2022 15.52, Maxim Levitsky wrote:
> On Thu, 2022-09-29 at 15:26 +0200, Paolo Bonzini wrote:
>> On 9/28/22 19:55, Sean Christopherson wrote:
>>>> As far as my opinion goes I do volunteer to test this code more often,
>>>> and I do not want to see the 32 bit KVM support be removed*yet*.
>>>
>>> Yeah, I 100% agree that it shouldn't be removed until we have equivalent test
>>> coverage.  But I do think it should an "off-by-default" sort of thing.  Maybe
>>> BROKEN is the wrong dependency though?  E.g. would EXPERT be a better option?
>>
>> Yeah, maybe EXPERT is better but I'm not sure of the equivalent test
>> coverage.  32-bit VMX/SVM kvm-unit-tests are surely a good idea, but
>> what's wrong with booting an older guest?
> 
>>>From my point of view, using the same kernel source for host and the guest
> is easier because you know that both kernels behave the same.
> 
> About EXPERT, IMHO these days most distros already dropped 32 bit suport thus anyway
> one needs to compile a recent 32 bit kernel manually - thus IMHO whoever
> these days compiles a 32 bit kernel, knows what they are doing.
> 
> I personally would wait few more releases when there is a pressing reason to remove
> this support.

FWIW, from the QEMU perspective, it would be very helpful to remove 32-bit 
KVM support from the kernel. The QEMU project currently struggles badly with 
keeping everything tested in the CI in a reasonable amount of time. The 
32-bit KVM kernel support is the only reason to keep the qemu-system-i386 
binary around - everything else can be covered with the qemu-system-x86_64 
binary that is a superset of the -i386 variant (except for the KVM part as 
far as I know).
Sure, we could also drop qemu-system-i386 from the CI without dropping the 
32-bit KVM code in the kernel, but I guess things will rather bitrot there 
even faster in that case, so I'd appreciate if the kernel could drop the 
32-bit in the near future, too.

  Thomas