Re: [RFC PATCH 1/4] powerpc: Optimize register usage for esr register

On Fri, Aug 6, 2021 at 2:53 PM Michael Ellerman [off-list ref] wrote:

sxwjean@me.com writes:

quoted

From: Xiongwei Song <redacted>

Create an anonymous union for dsisr and esr regsiters, we can reference
esr to get the exception detail when CONFIG_4xx=y or CONFIG_BOOKE=y.
Otherwise, reference dsisr. This makes code more clear.

Signed-off-by: Xiongwei Song <redacted>
---
 arch/powerpc/include/asm/ptrace.h          |  5 ++++-
 arch/powerpc/include/uapi/asm/ptrace.h     |  5 ++++-
 arch/powerpc/kernel/process.c              |  2 +-
 arch/powerpc/kernel/ptrace/ptrace.c        |  2 ++
 arch/powerpc/kernel/traps.c                |  2 +-
 arch/powerpc/mm/fault.c                    | 16 ++++++++++++++--
 arch/powerpc/platforms/44x/machine_check.c |  4 ++--
 arch/powerpc/platforms/4xx/machine_check.c |  2 +-
 8 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/arch/powerpc/include/asm/ptrace.h b/arch/powerpc/include/asm/ptrace.h
index 3e5d470a6155..c252d04b1206 100644
--- a/arch/powerpc/include/asm/ptrace.h
+++ b/arch/powerpc/include/asm/ptrace.h

@@ -44,7 +44,10 @@ struct pt_regs
 #endif
                      unsigned long trap;
                      unsigned long dar;
-                     unsigned long dsisr;
+                     union {
+                             unsigned long dsisr;
+                             unsigned long esr;
+                     };

I don't mind doing that.

quoted

                      unsigned long result;
              };
      };

diff --git a/arch/powerpc/include/uapi/asm/ptrace.h b/arch/powerpc/include/uapi/asm/ptrace.h
index 7004cfea3f5f..e357288b5f34 100644
--- a/arch/powerpc/include/uapi/asm/ptrace.h
+++ b/arch/powerpc/include/uapi/asm/ptrace.h

@@ -53,7 +53,10 @@ struct pt_regs
      /* N.B. for critical exceptions on 4xx, the dar and dsisr
         fields are overloaded to hold srr0 and srr1. */
      unsigned long dar;              /* Fault registers */
-     unsigned long dsisr;            /* on 4xx/Book-E used for ESR */
+     union {
+             unsigned long dsisr;            /* on Book-S used for DSISR */
+             unsigned long esr;              /* on 4xx/Book-E used for ESR */
+     };
      unsigned long result;           /* Result of a system call */
 };

But I'm not sure about the use of anonymous unions in UAPI headers. Old
compilers don't support them, so there's a risk of breakage.

I'd rather we didn't touch the uapi version.

Ok. Will discard the change.

quoted

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 185beb290580..f74af8f9133c 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c

@@ -1499,7 +1499,7 @@ static void __show_regs(struct pt_regs *regs)
          trap == INTERRUPT_DATA_STORAGE ||
          trap == INTERRUPT_ALIGNMENT) {
              if (IS_ENABLED(CONFIG_4xx) || IS_ENABLED(CONFIG_BOOKE))
-                     pr_cont("DEAR: "REG" ESR: "REG" ", regs->dar, regs->dsisr);
+                     pr_cont("DEAR: "REG" ESR: "REG" ", regs->dar, regs->esr);
              else
                      pr_cont("DAR: "REG" DSISR: %08lx ", regs->dar, regs->dsisr);
      }

diff --git a/arch/powerpc/kernel/ptrace/ptrace.c b/arch/powerpc/kernel/ptrace/ptrace.c
index 0a0a33eb0d28..00789ad2c4a3 100644
--- a/arch/powerpc/kernel/ptrace/ptrace.c
+++ b/arch/powerpc/kernel/ptrace/ptrace.c

@@ -375,6 +375,8 @@ void __init pt_regs_check(void)
                   offsetof(struct user_pt_regs, dar));
      BUILD_BUG_ON(offsetof(struct pt_regs, dsisr) !=
                   offsetof(struct user_pt_regs, dsisr));
+     BUILD_BUG_ON(offsetof(struct pt_regs, esr) !=
+                  offsetof(struct user_pt_regs, esr));
      BUILD_BUG_ON(offsetof(struct pt_regs, result) !=
                   offsetof(struct user_pt_regs, result));

diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index dfbce527c98e..2164f5705a0b 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c

@@ -562,7 +562,7 @@ static inline int check_io_access(struct pt_regs *regs)
 #ifdef CONFIG_PPC_ADV_DEBUG_REGS
 /* On 4xx, the reason for the machine check or program exception
    is in the ESR. */
-#define get_reason(regs)     ((regs)->dsisr)
+#define get_reason(regs)     ((regs)->esr)
 #define REASON_FP            ESR_FP
 #define REASON_ILLEGAL               (ESR_PIL | ESR_PUO)
 #define REASON_PRIVILEGED    ESR_PPR

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index a8d0ce85d39a..62953d4e7c93 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c

@@ -541,7 +541,11 @@ static __always_inline void __do_page_fault(struct pt_regs *regs)
 {
      long err;

-     err = ___do_page_fault(regs, regs->dar, regs->dsisr);
+     if (IS_ENABLED(CONFIG_4xx) || IS_ENABLED(CONFIG_BOOKE))
+             err = ___do_page_fault(regs, regs->dar, regs->esr);
+     else
+             err = ___do_page_fault(regs, regs->dar, regs->dsisr);

As Christophe said, I don't thinks this is an improvement.

It makes the code less readable. If anyone is confused about what is
passed to ___do_page_fault() they can either read the comment above it,
or look at the definition of pt_regs to see that esr and dsisr share
storage.

Ok, thanks a lot.  Will send v2.

Regards,
Xiongwei

cheers