KVM: Add arch hooks for enabling/disabling virtualization

Add arch hooks that are invoked when KVM enables/disable virtualization.
x86 will use the hooks to register an "emergency disable" callback, which
is essentially an x86-specific shutdown notifier that is used when the
kernel is doing an emergency reboot/shutdown/kexec.

Add comments for the declarations to help arch code understand exactly
when the callbacks are invoked.  Alternatively, the APIs themselves could
communicate most of the same info, but kvm_arch_pre_enable_virtualization()
and kvm_arch_post_disable_virtualization() are a bit cumbersome, and make
it a bit less obvious that they are intended to be implemented as a pair.

Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Tested-by: Farrah Chen <farrah.chen@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240830043600.127750-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/linux/kvm_host.h

@@ -1521,6 +1521,20 @@ static inline void kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu) {}
 #endif
 
 #ifdef CONFIG_KVM_GENERIC_HARDWARE_ENABLING
+/*
+ * kvm_arch_{enable,disable}_virtualization() are called on one CPU, under
+ * kvm_usage_lock, immediately after/before 0=>1 and 1=>0 transitions of
+ * kvm_usage_count, i.e. at the beginning of the generic hardware enabling
+ * sequence, and at the end of the generic hardware disabling sequence.
+ */
+void kvm_arch_enable_virtualization(void);
+void kvm_arch_disable_virtualization(void);
+/*
+ * kvm_arch_{enable,disable}_virtualization_cpu() are called on "every" CPU to
+ * do the actual twiddling of hardware bits.  The hooks are called on all
+ * online CPUs when KVM enables/disabled virtualization, and on a single CPU
+ * when that CPU is onlined/offlined (including for Resume/Suspend).
+ */
 int kvm_arch_enable_virtualization_cpu(void);
 void kvm_arch_disable_virtualization_cpu(void);
 #endif

virt/kvm/kvm_main.c

@@ -5581,6 +5581,16 @@ static DEFINE_PER_CPU(bool, virtualization_enabled);
 static DEFINE_MUTEX(kvm_usage_lock);
 static int kvm_usage_count;
 
+__weak void kvm_arch_enable_virtualization(void)
+{
+
+}
+
+__weak void kvm_arch_disable_virtualization(void)
+{
+
+}
+
 static int kvm_enable_virtualization_cpu(void)
 {
 	if (__this_cpu_read(virtualization_enabled))
@@ -5680,6 +5690,8 @@ static int kvm_enable_virtualization(void)
 	if (kvm_usage_count++)
 		return 0;
 
+	kvm_arch_enable_virtualization();
+
 	r = cpuhp_setup_state(CPUHP_AP_KVM_ONLINE, "kvm/cpu:online",
 			      kvm_online_cpu, kvm_offline_cpu);
 	if (r)
@@ -5710,6 +5722,7 @@ static int kvm_enable_virtualization(void)
 	unregister_syscore_ops(&kvm_syscore_ops);
 	cpuhp_remove_state(CPUHP_AP_KVM_ONLINE);
 err_cpuhp:
+	kvm_arch_disable_virtualization();
 	--kvm_usage_count;
 	return r;
 }
@@ -5723,6 +5736,7 @@ static void kvm_disable_virtualization(void)
 
 	unregister_syscore_ops(&kvm_syscore_ops);
 	cpuhp_remove_state(CPUHP_AP_KVM_ONLINE);
+	kvm_arch_disable_virtualization();
 }
 
 static int kvm_init_virtualization(void)