Re: [PATCH 10/10] powerpc: move norestart trap flag to bit 0

[Christophe Leroy]

Le 15/03/2021 à 04:17, Nicholas Piggin a écrit :
Compact the trap flags down to use the low 4 bits of regs.trap.

A few 64e interrupt trap numbers set bit 4. Although they tended to be
trivial so it wasn't a real problem[1], it is not the right thing to do,
and confusing.

[*] E.g., 0x310 hypercall goes to unknown_exception, which prints
     regs->trap directly so 0x310 will appear fine, and only the syscall
     interrupt will test norestart, so it won't be confused by 0x310.

Signed-off-by: Nicholas Piggin <npig...@gmail.com>
---
  arch/powerpc/include/asm/ptrace.h | 14 ++++++++++----
  1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/include/asm/ptrace.h 
b/arch/powerpc/include/asm/ptrace.h
index 91194fdd5d01..6a04abfe5eb6 100644
--- a/arch/powerpc/include/asm/ptrace.h
+++ b/arch/powerpc/include/asm/ptrace.h
@@ -185,15 +185,21 @@ static inline void regs_set_return_value(struct pt_regs 
*regs, unsigned long rc)
  #define current_pt_regs() \
        ((struct pt_regs *)((unsigned long)task_stack_page(current) + 
THREAD_SIZE) - 1)
  
+/*
+ * The 4 low bits (0xf) are available as flags to overload the trap word,
+ * because interrupt vectors have minimum alignment of 0x10. TRAP_FLAGS_MASK
+ * must cover the bits used as flags, including bit 0 which is used as the
+ * "norestart" bit.
+ */
  #ifdef __powerpc64__
-#define TRAP_FLAGS_MASK                0x10
+#define TRAP_FLAGS_MASK                0x1
  #define TRAP(regs)            ((regs)->trap & ~TRAP_FLAGS_MASK)
  #else
  /*
   * On 4xx we use bit 1 in the trap word to indicate whether the exception
   * is a critical exception (1 means it is).
   */
-#define TRAP_FLAGS_MASK                0x1E
+#define TRAP_FLAGS_MASK                0xf

Could we set 0xf for all and remove the ifdef __powerpc64__ ?

  #define TRAP(regs)            ((regs)->trap & ~TRAP_FLAGS_MASK)
  #define IS_CRITICAL_EXC(regs) (((regs)->trap & 2) != 0)
  #define IS_MCHECK_EXC(regs)   (((regs)->trap & 4) != 0)
@@ -222,12 +228,12 @@ static inline bool trap_is_syscall(struct pt_regs *regs)
  
  static inline bool trap_norestart(struct pt_regs *regs)
  {
-       return regs->trap & 0x10;
+       return regs->trap & 0x1;
  }
  
  static inline void set_trap_norestart(struct pt_regs *regs)
  {
-       regs->trap |= 0x10;
+       regs->trap |= 0x1;
  }
  
  #define arch_has_single_step()	(1)


While we are playing with ->trap, in mm/book3s64/hash_utils.c there is an if (regs->trap == 0x400). 
Should be TRAP(regs) == 0x400 ?

Christophe