KVM: x86: Move kvm_rebooting to x86

Move kvm_rebooting, which is only read by x86, to KVM x86 so that it can
be moved again to core x86 code.  Add a "shutdown" arch hook to facilate
setting the flag in KVM x86, along with a pile of comments to provide more
context around what KVM x86 is doing and why.

Reviewed-by: Chao Gao <chao.gao@intel.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Sagi Shahar <sagis@google.com>
Link: https://patch.msgid.link/20260214012702.2368778-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

2 files changed, 23 insertions, 0 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a03530795707..7ac3578e6ec0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -700,6 +700,9 @@ static void drop_user_return_notifiers(void)
 		kvm_on_user_return(&msrs->urn);
 }
 
+__visible bool kvm_rebooting;
+EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_rebooting);
+
 /*
  * Handle a fault on a hardware virtualization (VMX or SVM) instruction.
  *
@@ -13177,6 +13180,25 @@ int kvm_arch_enable_virtualization_cpu(void)
 	return 0;
 }
 
+void kvm_arch_shutdown(void)
+{
+	/*
+	 * Set kvm_rebooting to indicate that KVM has asynchronously disabled
+	 * hardware virtualization, i.e. that errors and/or exceptions on SVM
+	 * and VMX instructions are expected and should be ignored.
+	 */
+	kvm_rebooting = true;
+
+	/*
+	 * Ensure kvm_rebooting is visible before IPIs are sent to other CPUs
+	 * to disable virtualization.  Effectively pairs with the reception of
+	 * the IPI (kvm_rebooting is read in task/exception context, but only
+	 * _needs_ to be read as %true after the IPI function callback disables
+	 * virtualization).
+	 */
+	smp_wmb();
+}
+
 void kvm_arch_disable_virtualization_cpu(void)
 {
 	kvm_x86_call(disable_virtualization_cpu)();
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 94d4f07aaaa0..b314649e5c02 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -54,6 +54,7 @@ struct kvm_host_values {
 	u64 arch_capabilities;
 };
 
+extern bool kvm_rebooting;
 void kvm_spurious_fault(void);
 
 #define SIZE_OF_MEMSLOTS_HASHTABLE \