[PATCH 09/20] ARC: entry: use gp to cache task pointer (vs. r25)

Vineet Gupta
2023-08-15T00:49:28Z

The motivation is eventual ABI considerations for ARCv3 but even without
it this change us worthwhile as diffstat reduces 100 net lines

r25 is a callee saved register, normally not saved by entry code in
pt_regs. However because of its usage in CONFIG_ARC_CURR_IN_REG it needs
to be. This in turn requires a whole bunch of special casing when we
need to access r25. Then there is distinction between user mode r25 vs.
kernel mode r25 - hence distinct SAVE_CALLEE_SAVED_{USER,KERNEL}

Instead use gp which is a scratch register and thus saved already in entry
code. This cleans things up significantly and much nocer on eyes:

- SAVE_CALLEE_SAVED_{USER,KERNEL} are now exactly same
- no special user_r25 slot in pt_reggs

Note that typical global asm registers are callee-saved (r25), but gp is
not callee-saved thus needs additional -ffixed-<reg> toggle

Signed-off-by: Vineet Gupta <vgupta@kernel.org>
---
arch/arc/Kconfig | 6 +-
arch/arc/Makefile | 6 +-
arch/arc/include/asm/current.h | 2 +-
arch/arc/include/asm/entry-arcv2.h | 17 ++---
arch/arc/include/asm/entry-compact.h | 35 +++------
arch/arc/include/asm/entry.h | 107 ++++++++-------------------
arch/arc/include/asm/ptrace.h | 6 +-
arch/arc/kernel/asm-offsets.c | 2 -
arch/arc/kernel/ctx_sw.c | 13 +---
arch/arc/kernel/ctx_sw_asm.S | 2 +-
arch/arc/kernel/entry.S | 3 +-
arch/arc/kernel/process.c | 11 ---
arch/arc/kernel/ptrace.c | 2 -
13 files changed, 58 insertions(+), 154 deletions(-)

diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index 47b4acc7d0c9..c92bacc1ff4c 100644
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -492,11 +492,11 @@ config ARC_KVADDR_SIZE
kernel-user gutter)

config ARC_CURR_IN_REG
- bool "Dedicate Register r25 for current_task pointer"
+ bool "cache current task pointer in gp"
default y
help
- This reserved Register R25 to point to Current Task in
- kernel mode. This saves memory access for each such access
+ This reserves gp register to point to Current Task in
+ kernel mode eliding memory access for each access

config ARC_EMUL_UNALIGNED
diff --git a/arch/arc/Makefile b/arch/arc/Makefile
index 329400a1c355..2390dd042e36 100644
--- a/arch/arc/Makefile
+++ b/arch/arc/Makefile
@@ -28,14 +28,14 @@ cflags-y += $(tune-mcpu-def-y)
endif
endif

-
ifdef CONFIG_ARC_CURR_IN_REG
# For a global register definition, make sure it gets passed to every file
# We had a customer reported bug where some code built in kernel was NOT using
-# any kernel headers, and missing the r25 global register
+# any kernel headers, and missing the global register
# Can't do unconditionally because of recursive include issues
# due to <linux/thread_info.h>
LINUXINCLUDE += -include $(srctree)/arch/arc/include/asm/current.h
+cflags-y += -ffixed-gp
endif

cflags-y += -fsection-anchors
@@ -67,7 +67,7 @@ cflags-$(CONFIG_ARC_DW2_UNWIND) += -fasynchronous-unwind-tables $(cfi)
# small data is default for elf32 tool-chain. If not usable, disable it
# This also allows repurposing GP as scratch reg to gcc reg allocator
disable_small_data := y
-cflags-$(disable_small_data) += -mno-sdata -fcall-used-gp
+cflags-$(disable_small_data) += -mno-sdata

cflags-$(CONFIG_CPU_BIG_ENDIAN) += -mbig-endian
ldflags-$(CONFIG_CPU_BIG_ENDIAN) += -EB
diff --git a/arch/arc/include/asm/current.h b/arch/arc/include/asm/current.h
index 9b9bdd3e6538..06be89f6f2f0 100644
--- a/arch/arc/include/asm/current.h
+++ b/arch/arc/include/asm/current.h
@@ -13,7 +13,7 @@

#ifdef CONFIG_ARC_CURR_IN_REG

-register struct task_struct *curr_arc asm("r25");
+register struct task_struct *curr_arc asm("gp");
#define current (curr_arc)

#else
diff --git a/arch/arc/include/asm/entry-arcv2.h b/arch/arc/include/asm/entry-arcv2.h
index 0ff4c0610561..858742feab71 100644
--- a/arch/arc/include/asm/entry-arcv2.h
+++ b/arch/arc/include/asm/entry-arcv2.h
@@ -18,7 +18,6 @@
* | orig_r0 |
* | event/ECR |
* | bta |
- * | user_r25 |
* | gp |
* | fp |
* | sp |
@@ -56,7 +55,7 @@
; hardware does even if CONFIG_ARC_IRQ_NO_AUTOSAVE
; 4. Auto save: (optional) r0-r11, blink, LPE,LPS,LPC, JLI,LDI,EI
;
- ; (B) Manually saved some regs: r12,r25,r30, sp,fp,gp, ACCL pair
+ ; (B) Manually saved some regs: r12,r30, sp,fp,gp, ACCL pair

#ifdef CONFIG_ARC_IRQ_NO_AUTOSAVE
; carve pt_regs on stack (case #3), PC/STAT32 already on stack
@@ -157,17 +156,17 @@

st r10, [sp, PT_sp] ; SP (pt_regs->sp)

-#ifdef CONFIG_ARC_CURR_IN_REG
- st r25, [sp, PT_user_r25]
- GET_CURR_TASK_ON_CPU r25
-#endif
-
#ifdef CONFIG_ARC_HAS_ACCL_REGS
ST2 r58, r59, PT_r58
#endif

/* clobbers r10, r11 registers pair */
DSP_SAVE_REGFILE_IRQ
+
+#ifdef CONFIG_ARC_CURR_IN_REG
+ GET_CURR_TASK_ON_CPU gp
+#endif
+
.endm

/*------------------------------------------------------------------------*/
@@ -188,10 +187,6 @@
sr r10, [AUX_USER_SP]
1:

-#ifdef CONFIG_ARC_CURR_IN_REG
- ld r25, [sp, PT_user_r25]
-#endif
-
/* clobbers r10, r11 registers pair */
DSP_RESTORE_REGFILE_IRQ

diff --git a/arch/arc/include/asm/entry-compact.h b/arch/arc/include/asm/entry-compact.h
index 67ff06e15cea..e3383e1cb040 100644
--- a/arch/arc/include/asm/entry-compact.h
+++ b/arch/arc/include/asm/entry-compact.h
@@ -151,14 +151,6 @@
/* ARC700 doesn't provide auto-stack switching */
SWITCH_TO_KERNEL_STK

-#ifdef CONFIG_ARC_CURR_IN_REG
- /* Treat r25 as scratch reg (save on stack) and load with "current" */
- PUSH r25
- GET_CURR_TASK_ON_CPU r25
-#else
- sub sp, sp, 4
-#endif
-
st.a r0, [sp, -8] /* orig_r0 needed for syscall (skip ECR slot) */
sub sp, sp, 4 /* skip pt_regs->sp, already saved above */

@@ -179,6 +171,11 @@

lr r10, [ecr]
st r10, [sp, PT_event] /* EV_Trap expects r10 to have ECR */
+
+#ifdef CONFIG_ARC_CURR_IN_REG
+ /* gp already saved on stack: now load with "current" */
+ GET_CURR_TASK_ON_CPU gp
+#endif
.endm

/*--------------------------------------------------------------
@@ -208,11 +205,8 @@
POP gp
RESTORE_R12_TO_R0

-#ifdef CONFIG_ARC_CURR_IN_REG
- ld r25, [sp, 12]
-#endif
ld sp, [sp] /* restore original sp */
- /* orig_r0, ECR, user_r25 skipped automatically */
+ /* orig_r0, ECR skipped automatically */
.endm

/* Dummy ECR values for Interrupts */
@@ -229,13 +223,6 @@

SWITCH_TO_KERNEL_STK

-#ifdef CONFIG_ARC_CURR_IN_REG
- /* Treat r25 as scratch reg (save on stack) and load with "current" */
- PUSH r25
- GET_CURR_TASK_ON_CPU r25
-#else
- sub sp, sp, 4
-#endif

PUSH 0x003\LVL\()abcd /* Dummy ECR */
sub sp, sp, 8 /* skip orig_r0 (not needed)
@@ -255,6 +242,10 @@
PUSHAX lp_start
PUSHAX bta_l\LVL\()

+#ifdef CONFIG_ARC_CURR_IN_REG
+ /* gp already saved on stack: now load with "current" */
+ GET_CURR_TASK_ON_CPU gp
+#endif
.endm

/*--------------------------------------------------------------
@@ -282,11 +273,7 @@
POP gp
RESTORE_R12_TO_R0

-#ifdef CONFIG_ARC_CURR_IN_REG
- ld r25, [sp, 12]
-#endif
- ld sp, [sp] /* restore original sp */
- /* orig_r0, ECR, user_r25 skipped automatically */
+ ld sp, [sp] /* restore original sp; orig_r0, ECR skipped implicitly */
.endm

/* Get thread_info of "current" tsk */
diff --git a/arch/arc/include/asm/entry.h b/arch/arc/include/asm/entry.h
index 2980bc9b7653..49c2e090cb5c 100644
--- a/arch/arc/include/asm/entry.h
+++ b/arch/arc/include/asm/entry.h
@@ -91,7 +91,7 @@
* Helpers to save/restore callee-saved regs:
* used by several macros below
*-------------------------------------------------------------*/
-.macro SAVE_R13_TO_R24
+.macro SAVE_R13_TO_R25
PUSH r13
PUSH r14
PUSH r15
@@ -104,9 +104,11 @@
PUSH r22
PUSH r23
PUSH r24
+ PUSH r25
.endm

-.macro RESTORE_R24_TO_R13
+.macro RESTORE_R25_TO_R13
+ POP r25
POP r24
POP r23
POP r22
@@ -121,81 +123,31 @@
POP r13
.endm

-/*--------------------------------------------------------------
- * Collect User Mode callee regs as struct callee_regs - needed by
- * fork/do_signal/unaligned-access-emulation.
- * (By default only scratch regs are saved on entry to kernel)
- *
- * Special handling for r25 if used for caching Task Pointer.
- * It would have been saved in task->thread.user_r25 already, but to keep
- * the interface same it is copied into regular r25 placeholder in
- * struct callee_regs.
- *-------------------------------------------------------------*/
+/*
+ * save user mode callee regs as struct callee_regs
+ * - needed by fork/do_signal/unaligned-access-emulation.
+ */
.macro SAVE_CALLEE_SAVED_USER
+ SAVE_R13_TO_R25
+.endm

- mov r12, sp ; save SP as ref to pt_regs
- SAVE_R13_TO_R24
-
-#ifdef CONFIG_ARC_CURR_IN_REG
- ; Retrieve orig r25 and save it with rest of callee_regs
- ld r12, [r12, PT_user_r25]
- PUSH r12
-#else
- PUSH r25
-#endif
-
+/*
+ * restore user mode callee regs as struct callee_regs
+ * - could have been changed by ptrace tracer or unaligned-access fixup
+ */
+.macro RESTORE_CALLEE_SAVED_USER
+ RESTORE_R25_TO_R13
.endm

-/*--------------------------------------------------------------
- * Save kernel Mode callee regs at the time of Contect Switch.
- *
- * Special handling for r25 if used for caching Task Pointer.
- * Kernel simply skips saving it since it will be loaded with
- * incoming task pointer anyways
- *-------------------------------------------------------------*/
+/*
+ * save/restore kernel mode callee regs at the time of context switch
+ */
.macro SAVE_CALLEE_SAVED_KERNEL
-
- SAVE_R13_TO_R24
-
-#ifdef CONFIG_ARC_CURR_IN_REG
- sub sp, sp, 4
-#else
- PUSH r25
-#endif
+ SAVE_R13_TO_R25
.endm

-/*--------------------------------------------------------------
- * Opposite of SAVE_CALLEE_SAVED_KERNEL
- *-------------------------------------------------------------*/
.macro RESTORE_CALLEE_SAVED_KERNEL
-
-#ifdef CONFIG_ARC_CURR_IN_REG
- add sp, sp, 4 /* skip usual r25 placeholder */
-#else
- POP r25
-#endif
- RESTORE_R24_TO_R13
-.endm
-
-/*--------------------------------------------------------------
- * Opposite of SAVE_CALLEE_SAVED_USER
- *
- * ptrace tracer or unaligned-access fixup might have changed a user mode
- * callee reg which is saved back to usual r25 storage location
- *-------------------------------------------------------------*/
-.macro RESTORE_CALLEE_SAVED_USER
-
-#ifdef CONFIG_ARC_CURR_IN_REG
- POP r12
-#else
- POP r25
-#endif
- RESTORE_R24_TO_R13
-
- ; SP is back to start of pt_regs
-#ifdef CONFIG_ARC_CURR_IN_REG
- st r12, [sp, PT_user_r25]
-#endif
+ RESTORE_R25_TO_R13
.endm

/*--------------------------------------------------------------
@@ -231,10 +183,10 @@

#ifdef CONFIG_SMP

-/*-------------------------------------------------
+/*
* Retrieve the current running task on this CPU
- * 1. Determine curr CPU id.
- * 2. Use it to index into _current_task[ ]
+ * - loads it from backing _current_task[] (and can't use the
+ * caching reg for current task
*/
.macro GET_CURR_TASK_ON_CPU reg
GET_CPU_ID \reg
@@ -256,7 +208,7 @@
add2 \tmp, @_current_task, \tmp
st \tsk, [\tmp]
#ifdef CONFIG_ARC_CURR_IN_REG
- mov r25, \tsk
+ mov gp, \tsk
#endif

.endm
@@ -271,21 +223,20 @@
.macro SET_CURR_TASK_ON_CPU tsk, tmp
st \tsk, [@_current_task]
#ifdef CONFIG_ARC_CURR_IN_REG
- mov r25, \tsk
+ mov gp, \tsk
#endif
.endm

#endif /* SMP / UNI */

-/* ------------------------------------------------------------------
+/*
* Get the ptr to some field of Current Task at @off in task struct
- * -Uses r25 for Current task ptr if that is enabled
+ * - Uses current task cached in reg if enabled
*/
-
#ifdef CONFIG_ARC_CURR_IN_REG

.macro GET_CURR_TASK_FIELD_PTR off, reg
- add \reg, r25, \off
+ add \reg, gp, \off
.endm

#else
diff --git a/arch/arc/include/asm/ptrace.h b/arch/arc/include/asm/ptrace.h
index cf90fcd2a628..e9798f46cdc4 100644
--- a/arch/arc/include/asm/ptrace.h
+++ b/arch/arc/include/asm/ptrace.h
@@ -52,11 +52,9 @@ struct pt_regs {
};
unsigned long event;
};
-
- unsigned long user_r25;
};

-#define MAX_REG_OFFSET offsetof(struct pt_regs, user_r25)
+#define MAX_REG_OFFSET offsetof(struct pt_regs, event)

#else

@@ -79,8 +77,6 @@ struct pt_regs {

unsigned long bta; /* bta_l1, bta_l2, erbta */

- unsigned long user_r25;
-
unsigned long r26; /* gp */
unsigned long fp;
unsigned long sp; /* user/kernel sp depending on where we came from */
diff --git a/arch/arc/kernel/asm-offsets.c b/arch/arc/kernel/asm-offsets.c
index 0e884036ab74..37324fd9a72f 100644
--- a/arch/arc/kernel/asm-offsets.c
+++ b/arch/arc/kernel/asm-offsets.c
@@ -63,8 +63,6 @@ int main(void)
DEFINE(PT_blink, offsetof(struct pt_regs, blink));
DEFINE(PT_lpe, offsetof(struct pt_regs, lp_end));
DEFINE(PT_lpc, offsetof(struct pt_regs, lp_count));
- DEFINE(PT_user_r25, offsetof(struct pt_regs, user_r25));
-
DEFINE(SZ_CALLEE_REGS, sizeof(struct callee_regs));
DEFINE(SZ_PT_REGS, sizeof(struct pt_regs));

diff --git a/arch/arc/kernel/ctx_sw.c b/arch/arc/kernel/ctx_sw.c
index bf16f777a0bc..40d89440b7e4 100644
--- a/arch/arc/kernel/ctx_sw.c
+++ b/arch/arc/kernel/ctx_sw.c
@@ -38,11 +38,7 @@ __switch_to(struct task_struct *prev_task, struct task_struct *next_task)
"st.a r22, [sp, -4] \n\t"
"st.a r23, [sp, -4] \n\t"
"st.a r24, [sp, -4] \n\t"
-#ifndef CONFIG_ARC_CURR_IN_REG
"st.a r25, [sp, -4] \n\t"
-#else
- "sub sp, sp, 4 \n\t" /* usual r25 placeholder */
-#endif

/* set ksp of outgoing task in tsk->thread.ksp */
#if KSP_WORD_OFF <= 255
@@ -58,7 +54,7 @@ __switch_to(struct task_struct *prev_task, struct task_struct *next_task)

/*
* setup _current_task with incoming tsk.
- * optionally, set r25 to that as well
+ * optionally, set caching reg to that as well
* For SMP extra work to get to &_current_task[cpu]
* (open coded SET_CURR_TASK_ON_CPU)
*/
@@ -72,19 +68,14 @@ __switch_to(struct task_struct *prev_task, struct task_struct *next_task)
"st %2, [r24] \n\t"
#endif
#ifdef CONFIG_ARC_CURR_IN_REG
- "mov r25, %2 \n\t"
+ "mov gp, %2 \n\t"
#endif

/* get ksp of incoming task from tsk->thread.ksp */
"ld.as sp, [%2, %1] \n\t"

/* start loading it's CALLEE reg file */
-
-#ifndef CONFIG_ARC_CURR_IN_REG
"ld.ab r25, [sp, 4] \n\t"
-#else
- "add sp, sp, 4 \n\t"
-#endif
"ld.ab r24, [sp, 4] \n\t"
"ld.ab r23, [sp, 4] \n\t"
"ld.ab r22, [sp, 4] \n\t"
diff --git a/arch/arc/kernel/ctx_sw_asm.S b/arch/arc/kernel/ctx_sw_asm.S
index 02c461484761..95cba6265e2b 100644
--- a/arch/arc/kernel/ctx_sw_asm.S
+++ b/arch/arc/kernel/ctx_sw_asm.S
@@ -49,7 +49,7 @@ __switch_to:
SET_CURR_TASK_ON_CPU r1, r3

/* reload SP with kernel mode stack pointer in task->thread.ksp */
- ld.as sp, [r1, (TASK_THREAD + THREAD_KSP)/4]
+ ld.as sp, [r1, KSP_WORD_OFF]

/* restore the registers */
RESTORE_CALLEE_SAVED_KERNEL
diff --git a/arch/arc/kernel/entry.S b/arch/arc/kernel/entry.S
index 54e91df678dd..cd26e0fa5044 100644
--- a/arch/arc/kernel/entry.S
+++ b/arch/arc/kernel/entry.S
@@ -210,7 +210,6 @@ trap_with_param:

; Save callee regs in case gdb wants to have a look
; SP will grow up by size of CALLEE Reg-File
- ; NOTE: clobbers r12
SAVE_CALLEE_SAVED_USER

; save location of saved Callee Regs @ thread_struct->pc
@@ -318,7 +317,7 @@ resume_user_mode_begin:
; tracer might call PEEKUSR(CALLEE reg)
;
; NOTE: SP will grow up by size of CALLEE Reg-File
- SAVE_CALLEE_SAVED_USER ; clobbers r12
+ SAVE_CALLEE_SAVED_USER

; save location of saved Callee Regs @ thread_struct->callee
GET_CURR_TASK_FIELD_PTR TASK_THREAD, r10
diff --git a/arch/arc/kernel/process.c b/arch/arc/kernel/process.c
index 980b71da2f61..96f591508142 100644
--- a/arch/arc/kernel/process.c
+++ b/arch/arc/kernel/process.c
@@ -162,7 +162,6 @@ asmlinkage void ret_from_fork(void);
* | SP |
* | orig_r0 |
* | event/ECR |
- * | user_r25 |
* ------------------ <===== END of PAGE
*/
int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
@@ -243,16 +242,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
*/
c_callee->r25 = task_thread_info(p)->thr_ptr;

-#ifdef CONFIG_ARC_CURR_IN_REG
- /*
- * setup usermode thread pointer #2:
- * however for this special use of r25 in kernel, __switch_to() sets
- * r25 for kernel needs and only in the final return path is usermode
- * r25 setup, from pt_regs->user_r25. So set that up as well
- */
- c_regs->user_r25 = c_callee->r25;
-#endif
-
return 0;
}

diff --git a/arch/arc/kernel/ptrace.c b/arch/arc/kernel/ptrace.c
index 2abdcd9b09e8..8226df3163fe 100644
--- a/arch/arc/kernel/ptrace.c
+++ b/arch/arc/kernel/ptrace.c
@@ -47,7 +47,6 @@ static const struct pt_regs_offset regoffset_table[] = {
REG_OFFSET_NAME(sp),
REG_OFFSET_NAME(orig_r0),
REG_OFFSET_NAME(event),
- REG_OFFSET_NAME(user_r25),
REG_OFFSET_END,
};

@@ -57,7 +56,6 @@ static const struct pt_regs_offset regoffset_table[] = {
REG_OFFSET_NAME(orig_r0),
REG_OFFSET_NAME(event),
REG_OFFSET_NAME(bta),
- REG_OFFSET_NAME(user_r25),
REG_OFFSET_NAME(r26),
REG_OFFSET_NAME(fp),
REG_OFFSET_NAME(sp),
--
2.34.1