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

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Message-ID: <CAHk-=wiQ2m+zkBUhb1m=m6S-H1syAgWmCHzit9=5y7XsriKFvw@mail.gmail.com>
Date: Sun, 29 Sep 2024 10:35:59 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Kai Huang <kai.huang@...el.com>, Chao Gao <chao.gao@...el.com>, 
	Farrah Chen <farrah.chen@...el.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [GIT PULL] KVM/x86 changes for Linux 6.12

On Sat, 28 Sept 2024 at 08:33, Paolo Bonzini <pbonzini@...hat.com> 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

View attachment "patch.diff" of type "text/x-patch" (794 bytes)