Re: [PATCH] KVM: SVM: Add exception to disable objtool warning for kvm-amd.o

[Paolo Bonzini <pbonzini@redhat.com>]

On 8/5/23 02:55, Peter Zijlstra wrote:
>> +	 * Clobbering BP here is mostly ok since GIF will block NMIs and with
>> +	 * the exception of #MC and the kvm_rebooting _ASM_EXTABLE()s below
>> +	 * nothing untoward will happen until BP is restored.
>> +	 *
>> +	 * The kvm_rebooting exceptions should not want to unwind stack, and
>> +	 * while #MV might want to unwind stack, it is ultimately fatal.
>> +	 */
> Aside from me not being able to type #MC, I did realize that the
> kvm_reboot exception will go outside noinstr code and can hit
> tracing/instrumentation and do unwinds from there.

Asynchronously disabling SVM requires an IPI, so kvm_rebooting cannot 
change within CLGI/STGI.   We can check it after CLGI instead of waiting 
for a #GP:

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 956726d867aa..e3755f5eaf81 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4074,7 +4074,10 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct 
kvm_vcpu *vcpu)
  	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
  		x86_spec_ctrl_set_guest(svm->virt_spec_ctrl);

-	svm_vcpu_enter_exit(vcpu, spec_ctrl_intercepted);
+	if (unlikely(kvm_rebooting))
+		svm->vmcb->control.exit_code = SVM_EXIT_PAUSE;
+	else
+		svm_vcpu_enter_exit(vcpu, spec_ctrl_intercepted);

  	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
  		x86_spec_ctrl_restore_host(svm->virt_spec_ctrl);
diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
index 8e8295e774f0..34641b3a6823 100644
--- a/arch/x86/kvm/svm/vmenter.S
+++ b/arch/x86/kvm/svm/vmenter.S
@@ -270,23 +270,12 @@ SYM_FUNC_START(__svm_vcpu_run)
  	RESTORE_GUEST_SPEC_CTRL_BODY
  	RESTORE_HOST_SPEC_CTRL_BODY

-10:	cmpb $0, kvm_rebooting
-	jne 2b
-	ud2
-30:	cmpb $0, kvm_rebooting
-	jne 4b
-	ud2
-50:	cmpb $0, kvm_rebooting
-	jne 6b
-	ud2
-70:	cmpb $0, kvm_rebooting
-	jne 8b
-	ud2
+10:	ud2

  	_ASM_EXTABLE(1b, 10b)
-	_ASM_EXTABLE(3b, 30b)
-	_ASM_EXTABLE(5b, 50b)
-	_ASM_EXTABLE(7b, 70b)
+	_ASM_EXTABLE(3b, 10b)
+	_ASM_EXTABLE(5b, 10b)
+	_ASM_EXTABLE(7b, 10b)

  SYM_FUNC_END(__svm_vcpu_run)


Paolo