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

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Date:   Wed, 28 Sep 2022 17:55:17 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...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, Maxim Levitsky wrote:
> 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.

Oh, I didn't mean that it actually requires a lot of attention in terms of bug
fixes, what I meant by "maintenance cost" is the cost of testing that all the
flavors of 32-bit KVM actually work.  That can be automated to some extent, but
there's a non-trivial cost to maintaining all that automation.

> 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?