[PATCH v10 27/38] x86/fred: Add a machine check entry stub for FRED

[Xin Li]

Like #DB, when occurred on different ring level, i.e., from user or kernel
context, #MCE needs to be handled on different stack: User #MCE on current
task stack, while kernel #MCE on a dedicated stack.

This is exactly how FRED event delivery invokes an exception handler: ring
3 event on level 0 stack, i.e., current task stack; ring 0 event on the
#MCE dedicated stack specified in the IA32_FRED_STKLVLS MSR. So unlike IDT,
the FRED machine check entry stub doesn't do stack switch.

Tested-by: Shan Kang <shan.k...@intel.com>
Signed-off-by: Xin Li <xin3...@intel.com>
---

Changes since v5:
* Disallow #DB inside #MCE for robustness sake (Peter Zijlstra).
---
 arch/x86/kernel/cpu/mce/core.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
index 6f35f724cc14..da0a4a102afe 100644
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -52,6 +52,7 @@
 #include <asm/mce.h>
 #include <asm/msr.h>
 #include <asm/reboot.h>
+#include <asm/fred.h>
 
 #include "internal.h"
 
@@ -2144,6 +2145,31 @@ DEFINE_IDTENTRY_MCE_USER(exc_machine_check)
        exc_machine_check_user(regs);
        local_db_restore(dr7);
 }
+
+#ifdef CONFIG_X86_FRED
+/*
+ * When occurred on different ring level, i.e., from user or kernel
+ * context, #MCE needs to be handled on different stack: User #MCE
+ * on current task stack, while kernel #MCE on a dedicated stack.
+ *
+ * This is exactly how FRED event delivery invokes an exception
+ * handler: ring 3 event on level 0 stack, i.e., current task stack;
+ * ring 0 event on the #MCE dedicated stack specified in the
+ * IA32_FRED_STKLVLS MSR. So unlike IDT, the FRED machine check entry
+ * stub doesn't do stack switch.
+ */
+DEFINE_FREDENTRY_MCE(exc_machine_check)
+{
+       unsigned long dr7;
+
+       dr7 = local_db_save();
+       if (user_mode(regs))
+               exc_machine_check_user(regs);
+       else
+               exc_machine_check_kernel(regs);
+       local_db_restore(dr7);
+}
+#endif
 #else
 /* 32bit unified entry point */
 DEFINE_IDTENTRY_RAW(exc_machine_check)
-- 
2.34.1