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

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Date:   Thu, 23 Feb 2023 08:01:08 +0100
From:   Thomas Huth <thuth@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        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 22/02/2023 23.27, Sean Christopherson wrote:
> On Fri, Feb 17, 2023, Thomas Huth wrote:
>> 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.
> 
> Ya, I would happily drop support for 32-bit kernels today, the only sticking point
> is the lack of 32-bit shadow paging test coverage, which unfortunately is a rather
> large point.  :-(

 From your point of view, would it be OK if QEMU dropped qemu-system-i386? 
I.e. would it be fine to use older versions of QEMU only for that test 
coverage (or do you even use a different userspace for testing that)?

  Thomas