x86: defer processing events on the NMI exit path

Otherwise, we may end up in the scheduler, keeping NMIs masked for a
possibly unbounded period of time (until whenever the next IRET gets
executed). Enforce timely event processing by sending a self IPI.

Of course it's open for discussion whether to always use the straight
exit path from handle_ist_exception.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Send IPI to ourselves to enforce event processing. Dropped all ACKs.

--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -171,7 +171,7 @@ compat_bad_hypercall:
         jmp  compat_test_all_events
 
 /* %rbx: struct vcpu, interrupts disabled */
-compat_restore_all_guest:
+ENTRY(compat_restore_all_guest)
         ASSERT_INTERRUPTS_DISABLED
         RESTORE_ALL adj=8 compat=1
 .Lft0:  iretq
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -11,6 +11,7 @@
 #include <asm/apicdef.h>
 #include <asm/page.h>
 #include <public/xen.h>
+#include <irq_vectors.h>
 
         ALIGN
 /* %rbx: struct vcpu */
@@ -635,6 +636,9 @@ ENTRY(early_page_fault)
         jmp   restore_all_xen
         .popsection
 
+ENTRY(nmi)
+        pushq $0
+        movl  $TRAP_nmi,4(%rsp)
 handle_ist_exception:
         SAVE_ALL
         testb $3,UREGS_cs(%rsp)
@@ -649,12 +653,25 @@ handle_ist_exception:
         movzbl UREGS_entry_vector(%rsp),%eax
         leaq  exception_table(%rip),%rdx
         callq *(%rdx,%rax,8)
-        jmp   ret_from_intr
+        cmpb  $TRAP_nmi,UREGS_entry_vector(%rsp)
+        jne   ret_from_intr
 
-ENTRY(nmi)
-        pushq $0
-        movl  $TRAP_nmi,4(%rsp)
-        jmp   handle_ist_exception
+        /* We want to get straight to the IRET on the NMI exit path. */
+        testb $3,UREGS_cs(%rsp)
+        jz    restore_all_xen
+        GET_CURRENT(%rbx)
+        /* Send an IPI to ourselves to cover for the lack of event checking. */
+        movl  VCPU_processor(%rbx),%eax
+        shll  $IRQSTAT_shift,%eax
+        leaq  irq_stat(%rip),%rcx
+        cmpl  $0,(%rcx,%rax,1)
+        je    1f
+        movl  $EVENT_CHECK_VECTOR,%edi
+        call  send_IPI_self
+1:      movq  VCPU_domain(%rbx),%rax
+        cmpb  $0,DOMAIN_is_32bit_pv(%rax)
+        je    restore_all_guest
+        jmp   compat_restore_all_guest
 
 ENTRY(nmi_crash)
         pushq $0