[PATCH v5 34/34] KVM: x86/vmx: execute "int $2" to handle NMI in NMI caused VM exits when FRED is enabled

[Xin Li]

Execute "int $2" to handle NMI in NMI caused VM exits when FRED is enabled.

Like IRET for IDT, ERETS/ERETU are required to end the NMI handler for FRED
to unblock NMI ASAP (w/ bit 28 of CS set). And there are 2 approaches to
invoke the FRED NMI handler:
1) execute "int $2", let the h/w do the job.
2) create a FRED NMI stack frame on the current kernel stack with ASM,
   and then jump to fred_entrypoint_kernel in arch/x86/entry/entry_64_fred.S.

1) is preferred as we want less ASM.

Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
---

Changes since v4:
*) Do NOT use the term "injection", which in the KVM context means to
   reinject an event into the guest (Sean Christopherson).
*) Add the explanation of why to execute "int $2" to invoke the NMI handler
   in NMI caused VM exits (Sean Christopherson).
---
 arch/x86/kvm/vmx/vmx.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 3ebeaab34b2e..4f12ead2266b 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7229,7 +7229,16 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 	if ((u16)vmx->exit_reason.basic == EXIT_REASON_EXCEPTION_NMI &&
 	    is_nmi(vmx_get_intr_info(vcpu))) {
 		kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
-		vmx_do_nmi_irqoff();
+		/*
+		 * Like IRET for IDT, ERETS/ERETU are required to end the NMI
+		 * handler for FRED to unblock NMI ASAP (w/ bit 28 of CS set).
+		 *
+		 * Invoke the FRED NMI handler through executing "int $2".
+		 */
+		if (cpu_feature_enabled(X86_FEATURE_FRED))
+			asm volatile("int $2");
+		else
+			vmx_do_nmi_irqoff();
 		kvm_after_interrupt(vcpu);
 	}
 
-- 
2.34.1