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Subject: Re: [PATCH] KVM: x86: disable on 32-bit unless CONFIG_BROKEN
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Date:   Wed, 28 Sep 2022 20:44:22 +0300
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References: <20220926165112.603078-1-pbonzini@redhat.com>
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         <ed74c9a9d6a0d2fd2ad8bd98214ad36e97c243a0.camel@redhat.com>
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On Wed, 2022-09-28 at 16:12 +0000, Sean Christopherson wrote:
> On Wed, Sep 28, 2022, Paolo Bonzini wrote:
> > On 9/28/22 09:10, Maxim Levitsky wrote:
> > > I also think that outside KVM developers nobody should be using KVM on 32 bit host.
> > > 
> > > However for_developement_  I think that 32 bit KVM support is very useful, as it
> > > allows to smoke test the support for 32 bit nested hypervisors, which I do once in a while,
> > > and can even probably be useful to some users (e.g running some legacy stuff in a VM,
> > > which includes a hypervisor, especially to run really legacy OSes / custom bare metal software,
> > > using an old hypervisor) - or in other words, 32 bit nested KVM is mostly useless, but
> > > other 32 bit nested hypervisors can be useful.
> > > 
> > > Yes, I can always use an older 32 bit kernel in a guest with KVM support, but as long
> > > as current kernel works, it is useful to use the same kernel on host and guest.
> > 
> > Yeah, I would use older 32 bit kernels just like I use RHEL4 to test PIT
> > reinjection. :)  But really the ultimate solution to this would be to
> > improve kvm-unit-tests so that we can compile vmx.c and svm.c for 32-bit.
> 
> Agreed.  I too use 32-bit KVM to validate KVM's handling of 32-bit L1 hypervisors,
> but the maintenance cost is painfully high.
> 

But is it actually? I test it routinely and it it does work quite well IMHO.
I don't remember that there were that much breakage lately in this area.

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*.

Best regards,
	Maxim Levitsky