x86/debug: Remove the historical junk

Remove the historical junk and replace it with a WARN and a comment.

The problem is that even though the kernel only uses TF single-step in
kprobes and KGDB, both of which consume the event before this, QEMU/KVM has
bugs in this area that can trigger this state so it has to be dealt with.
Suggested-by: Brian Gerst <brgerst@gmail.com>
Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20200902133201.170216274@infradead.org

arch/x86/kernel/traps.c

@@ -843,18 +843,19 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
 	if (notify_debug(regs, &dr6))
 		goto out;
 
-	if (WARN_ON_ONCE(dr6 & DR_STEP)) {
 	/*
-		 * Historical junk that used to handle SYSENTER single-stepping.
-		 * This should be unreachable now.  If we survive for a while
-		 * without anyone hitting this warning, we'll turn this into
-		 * an oops.
+	 * The kernel doesn't use TF single-step outside of:
+	 *
+	 *  - Kprobes, consumed through kprobe_debug_handler()
+	 *  - KGDB, consumed through notify_debug()
+	 *
+	 * So if we get here with DR_STEP set, something is wonky.
+	 *
+	 * A known way to trigger this is through QEMU's GDB stub,
+	 * which leaks #DB into the guest and causes IST recursion.
 	 */
-		dr6 &= ~DR_STEP;
-		set_thread_flag(TIF_SINGLESTEP);
+	if (WARN_ON_ONCE(current->thread.debugreg6 & DR_STEP))
 		regs->flags &= ~X86_EFLAGS_TF;
-	}
-
 out:
 	instrumentation_end();
 	idtentry_exit_nmi(regs, irq_state);