[PATCH 11/20] ARC: __switch_to: asm with dwarf ops (vs. inline asm)

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

__switch_to() is final step of context switch, swapping kernel modes
stack (and callee regs) of outgoing task with next task.

It is also the starting point of stack unwinging of a sleeping task and
captures SP, FP, BLINK and the corresponding dwarf info. Back when
dinosaurs still roamed around, ARC gas didn't support CFI pseudo ops and
gcc was responsible for generating dwarf info. Thus it had to be written
in "C" with inline asm to do the hand crafting of stack. The function
prologue (and crucial saving of blink etc) was still gcc generated but
not visible in code. Likewise dwarf info was missing.

Now with modern tools, we can make things more obvious by writing the
code in asm and adding approproate dwarf cfi pseudo ops.

This is mostly non functional change, except for slight chnages to asm

- ARCompact doesn't support MOV_S fp, sp, so we use MOV

Signed-off-by: Vineet Gupta <vgupta@kernel.org>
---
arch/arc/include/asm/dwarf.h | 32 +++++++----
arch/arc/kernel/Makefile | 9 +--
arch/arc/kernel/ctx_sw.c | 103 -----------------------------------
arch/arc/kernel/ctx_sw_asm.S | 72 +++++++++++++-----------
4 files changed, 61 insertions(+), 155 deletions(-)
delete mode 100644 arch/arc/kernel/ctx_sw.c

diff --git a/arch/arc/include/asm/dwarf.h b/arch/arc/include/asm/dwarf.h
index 5f4de05bd4ee..a0d5ebe1bc3f 100644
--- a/arch/arc/include/asm/dwarf.h
+++ b/arch/arc/include/asm/dwarf.h
@@ -10,23 +10,31 @@

#ifdef ARC_DW2_UNWIND_AS_CFI

-#define CFI_STARTPROC .cfi_startproc
-#define CFI_ENDPROC .cfi_endproc
-#define CFI_DEF_CFA .cfi_def_cfa
-#define CFI_REGISTER .cfi_register
-#define CFI_REL_OFFSET .cfi_rel_offset
-#define CFI_UNDEFINED .cfi_undefined
+#define CFI_STARTPROC .cfi_startproc
+#define CFI_ENDPROC .cfi_endproc
+#define CFI_DEF_CFA .cfi_def_cfa
+#define CFI_DEF_CFA_OFFSET .cfi_def_cfa_offset
+#define CFI_DEF_CFA_REGISTER .cfi_def_cfa_register
+#define CFI_OFFSET .cfi_offset
+#define CFI_REL_OFFSET .cfi_rel_offset
+#define CFI_REGISTER .cfi_register
+#define CFI_RESTORE .cfi_restore
+#define CFI_UNDEFINED .cfi_undefined

#else

#define CFI_IGNORE #

-#define CFI_STARTPROC CFI_IGNORE
-#define CFI_ENDPROC CFI_IGNORE
-#define CFI_DEF_CFA CFI_IGNORE
-#define CFI_REGISTER CFI_IGNORE
-#define CFI_REL_OFFSET CFI_IGNORE
-#define CFI_UNDEFINED CFI_IGNORE
+#define CFI_STARTPROC CFI_IGNORE
+#define CFI_ENDPROC CFI_IGNORE
+#define CFI_DEF_CFA CFI_IGNORE
+#define CFI_DEF_CFA_OFFSET CFI_IGNORE
+#define CFI_DEF_CFA_REGISTER CFI_IGNORE
+#define CFI_OFFSET CFI_IGNORE
+#define CFI_REL_OFFSET CFI_IGNORE
+#define CFI_REGISTER CFI_IGNORE
+#define CFI_RESTORE CFI_IGNORE
+#define CFI_UNDEFINED CFI_IGNORE

#endif /* !ARC_DW2_UNWIND_AS_CFI */

diff --git a/arch/arc/kernel/Makefile b/arch/arc/kernel/Makefile
index 0723d888ac44..95fbf9364c67 100644
--- a/arch/arc/kernel/Makefile
+++ b/arch/arc/kernel/Makefile
@@ -5,6 +5,8 @@

obj-y := head.o arcksyms.o setup.o irq.o reset.o ptrace.o process.o devtree.o
obj-y += signal.o traps.o sys.o troubleshoot.o stacktrace.o disasm.o
+obj-y += ctx_sw_asm.o
+
obj-$(CONFIG_ISA_ARCOMPACT) += entry-compact.o intc-compact.o
obj-$(CONFIG_ISA_ARCV2) += entry-arcv2.o intc-arcv2.o

@@ -24,11 +26,4 @@ ifdef CONFIG_ISA_ARCOMPACT
CFLAGS_fpu.o += -mdpfp
endif

-ifdef CONFIG_ARC_DW2_UNWIND
-CFLAGS_ctx_sw.o += -fno-omit-frame-pointer
-obj-y += ctx_sw.o
-else
-obj-y += ctx_sw_asm.o
-endif
-
extra-y := vmlinux.lds
diff --git a/arch/arc/kernel/ctx_sw.c b/arch/arc/kernel/ctx_sw.c
deleted file mode 100644
index 40d89440b7e4..000000000000
--- a/arch/arc/kernel/ctx_sw.c
+++ /dev/null
@@ -1,103 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0-only
-/*
- * Copyright (C) 2004, 2007-2010, 2011-2012 Synopsys, Inc. (www.synopsys.com)
- *
- * Vineetg: Aug 2009
- * -"C" version of lowest level context switch asm macro called by schedular
- * gcc doesn't generate the dward CFI info for hand written asm, hence can't
- * backtrace out of it (e.g. tasks sleeping in kernel).
- * So we cheat a bit by writing almost similar code in inline-asm.
- * -This is a hacky way of doing things, but there is no other simple way.
- * I don't want/intend to extend unwinding code to understand raw asm
- */
-
-#include <asm/asm-offsets.h>
-#include <asm/switch_to.h>
-#include <linux/sched/debug.h>
-
-#define KSP_WORD_OFF ((TASK_THREAD + THREAD_KSP) / 4)
-
-struct task_struct *__sched
-__switch_to(struct task_struct *prev_task, struct task_struct *next_task)
-{
- unsigned int tmp;
- unsigned int prev = (unsigned int)prev_task;
- unsigned int next = (unsigned int)next_task;
-
- __asm__ __volatile__(
- /* FP/BLINK save generated by gcc (standard function prologue */
- "st.a r13, [sp, -4] \n\t"
- "st.a r14, [sp, -4] \n\t"
- "st.a r15, [sp, -4] \n\t"
- "st.a r16, [sp, -4] \n\t"
- "st.a r17, [sp, -4] \n\t"
- "st.a r18, [sp, -4] \n\t"
- "st.a r19, [sp, -4] \n\t"
- "st.a r20, [sp, -4] \n\t"
- "st.a r21, [sp, -4] \n\t"
- "st.a r22, [sp, -4] \n\t"
- "st.a r23, [sp, -4] \n\t"
- "st.a r24, [sp, -4] \n\t"
- "st.a r25, [sp, -4] \n\t"
-
- /* set ksp of outgoing task in tsk->thread.ksp */
-#if KSP_WORD_OFF <= 255
- "st.as sp, [%3, %1] \n\t"
-#else
- /*
- * Workaround for NR_CPUS=4k
- * %1 is bigger than 255 (S9 offset for st.as)
- */
- "add2 r24, %3, %1 \n\t"
- "st sp, [r24] \n\t"
-#endif
-
- /*
- * setup _current_task with incoming tsk.
- * 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)
- */
-#ifndef CONFIG_SMP
- "st %2, [@_current_task] \n\t"
-#else
- "lr r24, [identity] \n\t"
- "lsr r24, r24, 8 \n\t"
- "bmsk r24, r24, 7 \n\t"
- "add2 r24, @_current_task, r24 \n\t"
- "st %2, [r24] \n\t"
-#endif
-#ifdef CONFIG_ARC_CURR_IN_REG
- "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 */
- "ld.ab r25, [sp, 4] \n\t"
- "ld.ab r24, [sp, 4] \n\t"
- "ld.ab r23, [sp, 4] \n\t"
- "ld.ab r22, [sp, 4] \n\t"
- "ld.ab r21, [sp, 4] \n\t"
- "ld.ab r20, [sp, 4] \n\t"
- "ld.ab r19, [sp, 4] \n\t"
- "ld.ab r18, [sp, 4] \n\t"
- "ld.ab r17, [sp, 4] \n\t"
- "ld.ab r16, [sp, 4] \n\t"
- "ld.ab r15, [sp, 4] \n\t"
- "ld.ab r14, [sp, 4] \n\t"
- "ld.ab r13, [sp, 4] \n\t"
-
- /* last (ret value) = prev : although for ARC it mov r0, r0 */
- "mov %0, %3 \n\t"
-
- /* FP/BLINK restore generated by gcc (standard func epilogue */
-
- : "=r"(tmp)
- : "n"(KSP_WORD_OFF), "r"(next), "r"(prev)
- : "blink"
- );
-
- return (struct task_struct *)tmp;
-}
diff --git a/arch/arc/kernel/ctx_sw_asm.S b/arch/arc/kernel/ctx_sw_asm.S
index 95cba6265e2b..59d779004e64 100644
--- a/arch/arc/kernel/ctx_sw_asm.S
+++ b/arch/arc/kernel/ctx_sw_asm.S
@@ -13,48 +13,54 @@

#define KSP_WORD_OFF ((TASK_THREAD + THREAD_KSP) / 4)

-;################### Low Level Context Switch ##########################
+; IN
+; - r0: prev task (also current)
+; - r1: next task
+; OUT
+; - r0: prev task (so r0 not touched)

.section .sched.text,"ax",@progbits
- .align 4
- .global __switch_to
- .type __switch_to, @function
-__switch_to:
- CFI_STARTPROC
-
- /* Save regs on kernel mode stack of task */
- st.a blink, [sp, -4]
- st.a fp, [sp, -4]
+ENTRY_CFI(__switch_to)
+
+ /* save kernel stack frame regs of @prev task */
+ push blink
+ CFI_DEF_CFA_OFFSET 4
+ CFI_OFFSET r31, -4
+
+ push fp
+ CFI_DEF_CFA_OFFSET 8
+ CFI_OFFSET r27, -8
+
+ mov fp, sp
+ CFI_DEF_CFA_REGISTER r27
+
+ /* kernel mode callee regs of @prev */
SAVE_CALLEE_SAVED_KERNEL

- /* Save the now KSP in task->thread.ksp */
+ /* save final SP to @prev->thread.ksp */
#if KSP_WORD_OFF <= 255
st.as sp, [r0, KSP_WORD_OFF]
#else
/* Workaround for NR_CPUS=4k as ST.as can only take s9 offset */
- add2 r24, r0, KSP_WORD_OFF
- st sp, [r24]
+ add2 r10, r0, KSP_WORD_OFF
+ st sp, [r10]
#endif
- /*
- * Return last task in r0 (return reg)
- * On ARC, Return reg = First Arg reg = r0.
- * Since we already have last task in r0,
- * don't need to do anything special to return it
- */
-
- /*
- * switch to new task, contained in r1
- * Temp reg r3 is required to get the ptr to store val
- */
- SET_CURR_TASK_ON_CPU r1, r3
-
- /* reload SP with kernel mode stack pointer in task->thread.ksp */
- ld.as sp, [r1, KSP_WORD_OFF]
-
- /* restore the registers */
+ /* update @next in _current_task[] and GP register caching it */
+ SET_CURR_TASK_ON_CPU r1, r10
+
+ /* load SP from @next->thread.ksp */
+ ld.as sp, [r1, KSP_WORD_OFF]
+
+ /* restore callee regs, stack frame regs of @next */
RESTORE_CALLEE_SAVED_KERNEL
- ld.ab fp, [sp, 4]
- ld.ab blink, [sp, 4]
- j [blink]

+ pop fp
+ CFI_RESTORE r27
+ CFI_DEF_CFA r28, 4
+
+ pop blink
+ CFI_RESTORE r31
+ CFI_DEF_CFA_OFFSET 0
+
+ j [blink]
END_CFI(__switch_to)
--
2.34.1