x86/kexec: Stop hooking NMIs with trap_nop()

When FRED is active, it is not possible to hook NMIs like this.

NMI hooking in the crash path has undergone several revisions since its
introduction.  Notably since commit e7f147bf4a ("x86/crash: Drop manual
hooking of exception_table[]") we use the regular nmi_callback()
infrastructure.

Instead of asserting that we don't enter do_nmi_crash() on the crashing CPU,
tolerate it and return early.  It's a marginally longer codepath but behaves
the same and is compatible with FRED.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

xen/arch/x86/crash.c

@@ -37,14 +37,18 @@ static cpumask_t waiting_to_crash;
 static unsigned int crashing_cpu;
 static DEFINE_PER_CPU_READ_MOSTLY(bool, crash_save_done);
 
-/* This becomes the NMI handler for non-crashing CPUs, when Xen is crashing. */
-static int noreturn cf_check do_nmi_crash(
+/* This becomes the NMI handler for all CPUs when Xen is crashing. */
+static int cf_check do_nmi_crash(
     const struct cpu_user_regs *regs, int cpu)
 {
     stac();
 
-    /* nmi_shootdown_cpus() should ensure that this assertion is correct. */
-    ASSERT(cpu != crashing_cpu);
+    /*
+     * If we are the crashing CPU, do nothing.  We need to get back to the
+     * interrupted codepath to contine with the kexec transition.
+     */
+    if ( cpu == crashing_cpu )
+        return 1;
 
     /* Save crash information and shut down CPU.  Attempt only once. */
     if ( !this_cpu(crash_save_done) )
@@ -114,6 +118,8 @@ static int noreturn cf_check do_nmi_crash(
 
     for ( ; ; )
         halt();
+
+    unreachable();
 }
 
 static void nmi_shootdown_cpus(void)
@@ -130,11 +136,7 @@ static void nmi_shootdown_cpus(void)
 
     cpumask_andnot(&waiting_to_crash, &cpu_online_map, cpumask_of(cpu));
 
-    /*
-     * Disable IST for MCEs to avoid stack corruption race conditions, and
-     * change the NMI handler to a nop to avoid deviation from this codepath.
-     */
-    _set_gate_lower(&idt[X86_EXC_NMI], SYS_DESC_irq_gate, 0, &trap_nop);
+    /* Disable IST for MCEs to avoid stack corruption race conditions */
     set_ist(&idt[X86_EXC_MC], IST_NONE);
 
     set_nmi_callback(do_nmi_crash);