x86/entry/64, x86/nmi/64: Add CONFIG_DEBUG_ENTRY NMI testing code

It turns out to be rather tedious to test the NMI nesting code.
Make it easier: add a new CONFIG_DEBUG_ENTRY option that causes
the NMI handler to pre-emptively unmask NMIs.

With this option set, errors in the repeat_nmi logic or failures
to detect that we're in a nested NMI will result in quick panics
under perf (especially if multiple counters are running at high
frequency) instead of requiring an unusual workload that
generates page faults or breakpoints inside NMIs.

I called it CONFIG_DEBUG_ENTRY instead of CONFIG_DEBUG_NMI_ENTRY
because I want to add new non-NMI checks elsewhere in the entry
code in the future, and I'd rather not add too many new config
options or add this option and then immediately rename it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index a15893d..d8c0d32 100644 (file)
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -297,6 +297,18 @@ config OPTIMIZE_INLINING
 
          If unsure, say N.
 
+config DEBUG_ENTRY
+       bool "Debug low-level entry code"
+       depends on DEBUG_KERNEL
+       ---help---
+         This option enables sanity checks in x86's low-level entry code.
+         Some of these sanity checks may slow down kernel entries and
+         exits or otherwise impact performance.
+
+         This is currently used to help test NMI code.
+
+         If unsure, say N.
+
 config DEBUG_NMI_SELFTEST
        bool "NMI Selftest"
        depends on DEBUG_KERNEL && X86_LOCAL_APIC

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 5422bd2..8cb3e43 100644 (file)
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1454,6 +1454,21 @@ first_nmi:
 
        /* Everything up to here is safe from nested NMIs */
 
+#ifdef CONFIG_DEBUG_ENTRY
+       /*
+        * For ease of testing, unmask NMIs right away.  Disabled by
+        * default because IRET is very expensive.
+        */
+       pushq   $0              /* SS */
+       pushq   %rsp            /* RSP (minus 8 because of the previous push) */
+       addq    $8, (%rsp)      /* Fix up RSP */
+       pushfq                  /* RFLAGS */
+       pushq   $__KERNEL_CS    /* CS */
+       pushq   $1f             /* RIP */
+       INTERRUPT_RETURN        /* continues at repeat_nmi below */
+1:
+#endif
+
 repeat_nmi:
        /*
         * If there was a nested NMI, the first NMI's iret will return