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

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Message-ID: <YzRycXDnWgMDgbD7@google.com>
Date:   Wed, 28 Sep 2022 16:12:33 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: disable on 32-bit unless CONFIG_BROKEN

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.