x86/entry: Track the IST-ness of an entry for the exit paths

Use %r12 to hold an ist_exit boolean.  This register is zero elsewhere in the
entry/exit asm, so it only needs setting in the IST path.

As this is subtle and fragile, add check_ist_exit() to be used in debugging
builds to cross-check that the ist_exit boolean matches the entry vector.

Write check_ist_exit() it in C, because it's debug only and the logic more
complicated than I care to maintain in asm.

For now, we only need to use this signal in the exit-to-Xen path, but some
exit-to-guest paths happen in IST context too.  Check the correctness in all
exit paths to avoid the logic bit-rotting.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/traps.c

@@ -2259,6 +2259,19 @@ void asm_domain_crash_synchronous(unsigned long addr)
         do_softirq();
 }
 
+#ifdef CONFIG_DEBUG
+void check_ist_exit(const struct cpu_user_regs *regs, bool ist_exit)
+{
+    const unsigned int ist_mask =
+        (1U << X86_EXC_NMI) | (1U << X86_EXC_DB) |
+        (1U << X86_EXC_DF)  | (1U << X86_EXC_MC);
+    uint8_t ev = regs->entry_vector;
+    bool is_ist = (ev < X86_EXC_NUM) && ((1U << ev) & ist_mask);
+
+    ASSERT(is_ist == ist_exit);
+}
+#endif
+
 /*
  * Local variables:
  * mode: C

xen/arch/x86/x86_64/compat/entry.S

@@ -117,8 +117,15 @@ compat_process_trap:
         call  compat_create_bounce_frame
         jmp   compat_test_all_events
 
-/* %rbx: struct vcpu, interrupts disabled */
+/* %rbx: struct vcpu, %r12: ist_exit, interrupts disabled */
 ENTRY(compat_restore_all_guest)
+
+#ifdef CONFIG_DEBUG
+        mov   %rsp, %rdi
+        mov   %r12, %rsi
+        call  check_ist_exit
+#endif
+
         ASSERT_INTERRUPTS_DISABLED
         mov   $~(X86_EFLAGS_IOPL | X86_EFLAGS_VM), %r11d
         and   UREGS_eflags(%rsp),%r11d

xen/arch/x86/x86_64/entry.S

@@ -142,8 +142,15 @@ process_trap:
 
         .section .text.entry, "ax", @progbits
 
-/* %rbx: struct vcpu, interrupts disabled */
+/* %rbx: struct vcpu, %r12: ist_exit, interrupts disabled */
 restore_all_guest:
+
+#ifdef CONFIG_DEBUG
+        mov   %rsp, %rdi
+        mov   %r12, %rsi
+        call  check_ist_exit
+#endif
+
         ASSERT_INTERRUPTS_DISABLED
 
         /* Stash guest SPEC_CTRL value while we can read struct vcpu. */
@@ -659,8 +666,15 @@ ENTRY(early_page_fault)
         .section .text.entry, "ax", @progbits
 
         ALIGN
-/* No special register assumptions. */
+/* %r12=ist_exit */
 restore_all_xen:
+
+#ifdef CONFIG_DEBUG
+        mov   %rsp, %rdi
+        mov   %r12, %rsi
+        call  check_ist_exit
+#endif
+
         /*
          * Check whether we need to switch to the per-CPU page tables, in
          * case we return to late PV exit code (from an NMI or #MC).
@@ -1087,6 +1101,10 @@ handle_ist_exception:
 .L_ist_dispatch_done:
         mov   %r15, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
         mov   %bl, STACK_CPUINFO_FIELD(use_pv_cr3)(%r14)
+
+        /* This is an IST exit */
+        mov   $1, %r12d
+
         cmpb  $X86_EXC_NMI, UREGS_entry_vector(%rsp)
         jne   ret_from_intr