[PATCH v9 22/36] x86/fred: Add a double fault entry stub for FRED

[Xin Li <xin3.li@xxxxxxxxx>]

The IDT event delivery of a double fault pushes an error code into the
orig_ax member of the pt_regs structure, and the error code is passed
as the second argument of its C-handler exc_double_fault(), although
the pt_regs structure is already passed as the first argument.

The existing IDT double fault asm entry code does the following

  movq ORIG_RAX(%rsp), %rsi     /* get error code into 2nd argument*/
  movq $-1, ORIG_RAX(%rsp)      /* no syscall to restart */

to set the orig_ax member to -1 just before calling the C-handler.

X86_TRAP_TS, X86_TRAP_NP, X86_TRAP_SS, X86_TRAP_GP, X86_TRAP_AC and
X86_TRAP_CP are all handled in the same way because the IDT event
delivery pushes an error code into their stack frame for them.

The commit d99015b1abbad ("x86: move entry_64.S register saving out of
the macros") introduced the changes to set orig_ax to -1, but I can't
see why. Our tests with FRED seem fine if orig_ax is left unchanged
instead of set to -1. It's probably cleaner and simpler to remove the
second argument from exc_double_fault() while leave orig_ax unchanged
to pass the error code inside the first argument, at least on native
x86_64. That would be a separate, pre-FRED, patch.

For now just add a double fault entry stub for FRED, which simply
calls the existing exc_double_fault().

Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
---
 arch/x86/include/asm/fred.h | 1 +
 arch/x86/kernel/traps.c     | 7 +++++++
 2 files changed, 8 insertions(+)

diff --git a/arch/x86/include/asm/fred.h b/arch/x86/include/asm/fred.h
index f559dd9dc4f2..bd701ac87528 100644
--- a/arch/x86/include/asm/fred.h
+++ b/arch/x86/include/asm/fred.h
@@ -116,6 +116,7 @@ DECLARE_FRED_HANDLER(fred_exc_nmi);
 DECLARE_FRED_HANDLER(fred_exc_debug);
 DECLARE_FRED_HANDLER(fred_exc_page_fault);
 DECLARE_FRED_HANDLER(fred_exc_machine_check);
+DECLARE_FRED_HANDLER(fred_exc_double_fault);
 
 #endif /* __ASSEMBLY__ */
 
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index b10464966a81..49dd92458eb0 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -555,6 +555,13 @@ DEFINE_IDTENTRY_DF(exc_double_fault)
        instrumentation_end();
 }
 
+#ifdef CONFIG_X86_FRED
+DEFINE_FRED_HANDLER(fred_exc_double_fault)
+{
+       exc_double_fault(regs, regs->orig_ax);
+}
+#endif
+
 DEFINE_IDTENTRY(exc_bounds)
 {
        if (notify_die(DIE_TRAP, "bounds", regs, 0,
-- 
2.34.1