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

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Date:   Thu, 23 Feb 2023 10:33:08 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Thomas Huth <thuth@...hat.com>,
        Sean Christopherson <seanjc@...gle.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 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)?
> 
>   Thomas
> 

>From my point of view qemu-system-x86_64 does run 32 bit guests just fine.

The only exception that I know is that gdbstub is somewhat broken, but that can be probably
fixed.


Best regards,
	Maxim Levitsky