[RFC PATCH] powerpc: Investigate static_call concept
From: Christophe Leroy
Date: Fri Aug 27 2021 - 05:47:25 EST
This RFC is to validate the concept of static_call on powerpc.
Highly copied from x86.
It replaces ppc_md.get_irq() which is called at every IRQ, by
a static call.
When updating the call, we just replace the instruction at the
trampoline address by a relative jump to the function.
For the time being, the case of out-of-range functions is not handled.
Note that even if we don't immediately use static calls in powerpc
code, supporting static calls would immediately benefit to core
functionnalities using it, like tracing.
Tested on powerpc 8xx.
With the patch:
00000000 <__SCT__ppc_md_get_irq>:
0: 4e 80 00 20 blr <== Replaced by 'b <ppc_md.get_irq>' at runtime
...
00000038 <__do_irq>:
38: 94 21 ff f0 stwu r1,-16(r1)
3c: 7c 08 02 a6 mflr r0
40: 90 01 00 14 stw r0,20(r1)
44: 48 00 00 01 bl 44 <__do_irq+0xc>
44: R_PPC_REL24 __SCT__ppc_md_get_irq
...
Before the patch:
00000038 <__do_irq>:
38: 3d 20 00 00 lis r9,0
3a: R_PPC_ADDR16_HA ppc_md+0x20
3c: 94 21 ff f0 stwu r1,-16(r1)
40: 81 29 00 00 lwz r9,0(r9)
42: R_PPC_ADDR16_LO ppc_md+0x20
44: 7c 08 02 a6 mflr r0
48: 90 01 00 14 stw r0,20(r1)
4c: 7d 29 03 a6 mtctr r9
50: 4e 80 04 21 bctrl
...
Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---
arch/powerpc/Kconfig | 1 +
arch/powerpc/include/asm/machdep.h | 3 +++
arch/powerpc/include/asm/static_call.h | 25 +++++++++++++++++++++++++
arch/powerpc/kernel/Makefile | 2 +-
arch/powerpc/kernel/irq.c | 2 +-
arch/powerpc/kernel/setup-common.c | 4 ++++
arch/powerpc/kernel/static_call.c | 16 ++++++++++++++++
7 files changed, 51 insertions(+), 2 deletions(-)
create mode 100644 arch/powerpc/include/asm/static_call.h
create mode 100644 arch/powerpc/kernel/static_call.c
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 36b72d972568..c3930ea63e59 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -247,6 +247,7 @@ config PPC
select HAVE_SOFTIRQ_ON_OWN_STACK
select HAVE_STACKPROTECTOR if PPC32 && $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=r2)
select HAVE_STACKPROTECTOR if PPC64 && $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=r13)
+ select HAVE_STATIC_CALL
select HAVE_SYSCALL_TRACEPOINTS
select HAVE_VIRT_CPU_ACCOUNTING
select HUGETLB_PAGE_SIZE_VARIABLE if PPC_BOOK3S_64 && HUGETLB_PAGE
diff --git a/arch/powerpc/include/asm/machdep.h b/arch/powerpc/include/asm/machdep.h
index 764f2732a821..ac9712312b76 100644
--- a/arch/powerpc/include/asm/machdep.h
+++ b/arch/powerpc/include/asm/machdep.h
@@ -7,6 +7,7 @@
#include <linux/init.h>
#include <linux/dma-mapping.h>
#include <linux/export.h>
+#include <linux/static_call_types.h>
#include <asm/setup.h>
@@ -295,5 +296,7 @@ static inline void log_error(char *buf, unsigned int err_type, int fatal)
#define machine_late_initcall(mach, fn) __define_machine_initcall(mach, fn, 7)
#define machine_late_initcall_sync(mach, fn) __define_machine_initcall(mach, fn, 7s)
+DECLARE_STATIC_CALL(ppc_md_get_irq, *ppc_md.get_irq);
+
#endif /* __KERNEL__ */
#endif /* _ASM_POWERPC_MACHDEP_H */
diff --git a/arch/powerpc/include/asm/static_call.h b/arch/powerpc/include/asm/static_call.h
new file mode 100644
index 000000000000..335ee4ceaef9
--- /dev/null
+++ b/arch/powerpc/include/asm/static_call.h
@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_POWERPC_STATIC_CALL_H
+#define _ASM_POWERPC_STATIC_CALL_H
+
+#define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func) \
+ asm(".pushsection .text, \"ax\" \n" \
+ ".align 4 \n" \
+ ".globl " STATIC_CALL_TRAMP_STR(name) " \n" \
+ STATIC_CALL_TRAMP_STR(name) ": \n" \
+ " b " #func " \n" \
+ ".type " STATIC_CALL_TRAMP_STR(name) ", @function \n" \
+ ".size " STATIC_CALL_TRAMP_STR(name) ", . - " STATIC_CALL_TRAMP_STR(name) " \n" \
+ ".popsection \n")
+
+#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) \
+ asm(".pushsection .text, \"ax\" \n" \
+ ".align 4 \n" \
+ ".globl " STATIC_CALL_TRAMP_STR(name) " \n" \
+ STATIC_CALL_TRAMP_STR(name) ": \n" \
+ " blr \n" \
+ ".type " STATIC_CALL_TRAMP_STR(name) ", @function \n" \
+ ".size " STATIC_CALL_TRAMP_STR(name) ", . - " STATIC_CALL_TRAMP_STR(name) " \n" \
+ ".popsection \n")
+
+#endif /* _ASM_POWERPC_STATIC_CALL_H */
diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 7be36c1e1db6..08877252dff8 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -47,7 +47,7 @@ obj-y := cputable.o syscalls.o \
udbg.o misc.o io.o misc_$(BITS).o \
of_platform.o prom_parse.o firmware.o \
hw_breakpoint_constraints.o interrupt.o \
- kdebugfs.o
+ kdebugfs.o static_call.o
obj-y += ptrace/
obj-$(CONFIG_PPC64) += setup_64.o \
paca.o nvram_64.o note.o
diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index 551b653228c4..872f46e20754 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -736,7 +736,7 @@ void __do_irq(struct pt_regs *regs)
*
* This will typically lower the interrupt line to the CPU
*/
- irq = ppc_md.get_irq();
+ irq = static_call(ppc_md_get_irq)();
/* We can hard enable interrupts now to allow perf interrupts */
may_hard_irq_enable();
diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
index b1e43b69a559..57d06c163b7b 100644
--- a/arch/powerpc/kernel/setup-common.c
+++ b/arch/powerpc/kernel/setup-common.c
@@ -33,6 +33,7 @@
#include <linux/of_platform.h>
#include <linux/hugetlb.h>
#include <linux/pgtable.h>
+#include <linux/static_call.h>
#include <asm/io.h>
#include <asm/paca.h>
#include <asm/prom.h>
@@ -81,6 +82,8 @@ EXPORT_SYMBOL(ppc_md);
struct machdep_calls *machine_id;
EXPORT_SYMBOL(machine_id);
+DEFINE_STATIC_CALL_NULL(ppc_md_get_irq, *ppc_md.get_irq);
+
int boot_cpuid = -1;
EXPORT_SYMBOL_GPL(boot_cpuid);
@@ -613,6 +616,7 @@ void probe_machine(void)
machine_id++) {
DBG(" %s ...", machine_id->name);
memcpy(&ppc_md, machine_id, sizeof(struct machdep_calls));
+ static_call_update(ppc_md_get_irq, ppc_md.get_irq);
if (ppc_md.probe()) {
DBG(" match !\n");
break;
diff --git a/arch/powerpc/kernel/static_call.c b/arch/powerpc/kernel/static_call.c
new file mode 100644
index 000000000000..a281f1759c39
--- /dev/null
+++ b/arch/powerpc/kernel/static_call.c
@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/memory.h>
+#include <linux/static_call.h>
+
+#include <asm/code-patching.h>
+
+void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
+{
+ mutex_lock(&text_mutex);
+
+ if (tramp)
+ patch_branch(tramp, (unsigned long)func, 0);
+
+ mutex_unlock(&text_mutex);
+}
+EXPORT_SYMBOL_GPL(arch_static_call_transform);
--
2.25.0