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

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Date:   Fri, 24 Feb 2023 07:28:30 +0100
From:   Thomas Huth <thuth@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     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 23/02/2023 23.10, Sean Christopherson wrote:
> On Thu, Feb 23, 2023, Maxim Levitsky wrote:
>> On Thu, 2023-02-23 at 08:01 +0100, Thomas Huth wrote:
>>> 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)?
> 
> For me personally, I have no objection to dropping qemu-system-i386 support in
> future QEMU releases.  I update my 32-bit images very, very infrequently, so I
> probably wouldn't even notice for like 5 years :-)
> 
>>  From my point of view qemu-system-x86_64 does run 32 bit guests just fine.
> 
> Right, but unless I seriously misunderstand what qemu-system-x86_64 ecompasses,
> it can't be used to run guests of 32-bit _hosts_, which is what we need to test
> shadowing of 32-bit NPT.

That's what I've been told in the past, too, and that's why I asked. Thanks 
for the clarification!

To summarize: My takeaway is that nobody really needs qemu-system-i386 
anymore for recent development - the remaining 32-bit KVM use cases can be 
done with older versions of QEMU instead, thus it should be fine for the 
QEMU project to drop qemu-system-i386 nowadays.

  Thanks,
   Thomas