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

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Message-ID: <205b4d01-db86-6662-7906-24a8ca765e4f@redhat.com>
Date:   Thu, 29 Sep 2022 17:07:06 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: disable on 32-bit unless CONFIG_BROKEN

On 9/29/22 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.

It is certainly easier, but it is less correct.  You don't cover 
anything that KVM doesn't use.

> 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.
> 
> AFAIK, it is not really possible to remove most of the legacy direct mmu
> because shadowing mmu still can use it (I think Sean told me that once).

Yeah, it won't let us remove a lot of code but there are several logic 
cleanups that become possible if the TDP case can just assume the TDP 
MMU is there.  For example, there is no reason to have a cpu_role (as 
opposed to an mmu_page_role for the root) if you are building HPA->GPA 
page tables.

(Which reminds me that toggling CR0.WP is still a hog with the TDP MMU).

Paolo