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[Xen-devel] [PATCH V2] xen: vmx: Use an INT 2 call to process real NMI's instead of self_nmi() in VMEXIT handler

The self_nmi() code cause's an NMI to be triggered by sending an APIC message
to the local processor. Unfortunately there is a delay in the delivery of the
APIC message and we may already have re-entered the HVM guest by the time the
NMI is taken. This results in the VMEXIT code calling the self_nmi() function
again. We have seen hundreds of iterations of this VMEXIT/VMENTER loop before
the HVM guest resumes normal operation.

Volume 3 Chapter 27 Section 1 of the Intel SDM states:

An NMI causes subsequent NMIs to be blocked, but only after the VM exit

So we believe it is safe to directly invoke the INT call as NMI's should
already be blocked.

The INT 2 call will perform an IRET which will unblock later calls to the NMI
handler, according to Intel SDM Volume 3 Chapter 6.7.1. We must ensure that the
IRET from the INT 2 IRET is the first IRET issued to prevent losing an NMI.
Moving the INT 2 call to before the interrupts are enabled should ensure we
don't lose the NMI.

Signed-off-by: Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>
Acked-by: Tim Deegan <tim@xxxxxxx>

diff -r 62885b3c34c8 -r 7d6fd0219dd7 xen/arch/x86/hvm/vmx/vmx.c
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2269,6 +2269,14 @@ void vmx_vmexit_handler(struct cpu_user_
         vector = intr_info & INTR_INFO_VECTOR_MASK;
         if ( vector == TRAP_machine_check )
+        else if ( vector == TRAP_nmi &&
+                ( (intr_info & INTR_INFO_INTR_TYPE_MASK) ==
+                  (X86_EVENTTYPE_NMI << 8) ) )
+            /* Must be called before interrupts are enabled to ensure
+             * the NMI handler code is run before the first IRET. The
+             * IRET unblocks subsequent NMI's (Intel SDM Vol 3, 6.7.1)
+             */
+            asm volatile("int $2"); /* Real NMI, vector 2: normal processing */
@@ -2442,7 +2450,6 @@ void vmx_vmexit_handler(struct cpu_user_
                  (X86_EVENTTYPE_NMI << 8) )
                 goto exit_and_crash;
-            self_nmi(); /* Real NMI, vector 2: normal processing. */
         case TRAP_machine_check:

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