Re: [PATCH] KVM: x86: disable on 32-bit unless CONFIG_BROKEN

[Thomas Huth]

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