Re: [PATCH] x86: kvm: add back X86_LOCAL_APIC dependency

[Sean Christopherson <seanjc@google.com>]

On Tue, Nov 12, 2024, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> Enabling KVM now causes a build failure on x86-32 if X86_LOCAL_APIC
> is disabled:
> 
> arch/x86/kvm/svm/svm.c: In function 'svm_emergency_disable_virtualization_cpu':
> arch/x86/kvm/svm/svm.c:597:9: error: 'kvm_rebooting' undeclared (first use in this function); did you mean 'kvm_irq_routing'?
>   597 |         kvm_rebooting = true;
>       |         ^~~~~~~~~~~~~
>       |         kvm_irq_routing
> arch/x86/kvm/svm/svm.c:597:9: note: each undeclared identifier is reported only once for each function it appears in
> make[6]: *** [scripts/Makefile.build:221: arch/x86/kvm/svm/svm.o] Error 1
> In file included from include/linux/rculist.h:11,
>                  from include/linux/hashtable.h:14,
>                  from arch/x86/kvm/svm/avic.c:18:
> arch/x86/kvm/svm/avic.c: In function 'avic_pi_update_irte':
> arch/x86/kvm/svm/avic.c:909:38: error: 'struct kvm' has no member named 'irq_routing'
>   909 |         irq_rt = srcu_dereference(kvm->irq_routing, &kvm->irq_srcu);
>       |                                      ^~
> include/linux/rcupdate.h:538:17: note: in definition of macro '__rcu_dereference_check'
>   538 |         typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
> 
> Move the dependency to the same place as before.
> 
> Fixes: ea4290d77bda ("KVM: x86: leave kvm.ko out of the build if no vendor module is requested")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202410060426.e9Xsnkvi-lkp@intel.com/
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> Question: is there actually any point in keeping KVM support for 32-bit host
> processors?

Nope.  We need _a_ 32-bit KVM build to run as a nested (L1) hypervisor for testing
purposes, but AFAIK there's zero need to keep 32-bit KVM up-to-date.

> From what I can tell, the only 32-bit CPUs that support this are
> the rare Atom E6xx and Z5xx models and the even older Yonah/Sossaman "Core
> Duo", everything else is presumably better off just running a 64-bit kernel
> even for 32-bit guests?

Yep.  I am 99.9% certain there are no users of 32-bit KVM.  There have been
multiple instances in the past few years where 32-bit KVM was quite broken, for
several kernel releases, and no one complained.

Paolo, should we go ahead and start the process of removing 32-bit support in x86?
I forget who it was, but someone from the QEMU world mentioned that dropping 32-bit
support in KVM would allow dropping a rather large pile of code in QEMU too.