Re: [GIT PULL] KVM/x86 changes for Linux 6.12

[Linus Torvalds]

On Sat, 28 Sept 2024 at 08:33, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> Apologize for the late pull request; all the traveling made things a
> bit messy.  Also, we have a known regression here on ancient processors
> and will fix it next week.

.. actually, much worse than that, you have a build error.

  arch/x86/kvm/x86.c: In function ‘kvm_arch_enable_virtualization’:
  arch/x86/kvm/x86.c:12517:9: error: implicit declaration of function
‘cpu_emergency_register_virt_callback’
[-Wimplicit-function-declaration]
  12517 |
cpu_emergency_register_virt_callback(kvm_x86_ops.emergency_disable_virtualization_cpu);
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  arch/x86/kvm/x86.c: In function ‘kvm_arch_disable_virtualization’:
  arch/x86/kvm/x86.c:12522:9: error: implicit declaration of function
‘cpu_emergency_unregister_virt_callback’
[-Wimplicit-function-declaration]
  12522 |
cpu_emergency_unregister_virt_callback(kvm_x86_ops.emergency_disable_virtualization_cpu);
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

which I hadn't noticed before, because I did just allmodconfig builds.

But with a smaller config, the above error happens.

The culprit is commit 590b09b1d88e ("KVM: x86: Register "emergency
disable" callbacks when virt is enabled"), and the reason seems to be
this:

  #if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
  void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback);
  ...

ie if you have a config with KVM enabled, but neither KVM_INTEL nor
KVM_AMD set, you don't get that callback thing.

The fix may be something like the attached.

                   Linus
 arch/x86/include/asm/reboot.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/include/asm/reboot.h b/arch/x86/include/asm/reboot.h
index d0ef2a678d66..c02183d3cdd7 100644
--- a/arch/x86/include/asm/reboot.h
+++ b/arch/x86/include/asm/reboot.h
@@ -31,6 +31,8 @@ void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback);
 void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback);
 void cpu_emergency_disable_virtualization(void);
 #else
+static inline void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback) {}
+static inline void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback) {}
 static inline void cpu_emergency_disable_virtualization(void) {}
 #endif /* CONFIG_KVM_INTEL || CONFIG_KVM_AMD */