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Subject[PATCH v2 3/4] x86/static_call: Add out-of-line static call implementation
Date
Add the x86 out-of-line static call implementation.  For each key, a
permanent trampoline is created which is the destination for all static
calls for the given key. The trampoline has a direct jump which gets
patched by static_call_update() when the destination function changes.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
arch/x86/Kconfig | 1 +
arch/x86/include/asm/static_call.h | 28 ++++++++++++++++
arch/x86/kernel/Makefile | 1 +
arch/x86/kernel/static_call.c | 54 ++++++++++++++++++++++++++++++
include/linux/static_call.h | 2 +-
5 files changed, 85 insertions(+), 1 deletion(-)
create mode 100644 arch/x86/include/asm/static_call.h
create mode 100644 arch/x86/kernel/static_call.c

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b5286ad2a982..a2a10e0ce248 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -189,6 +189,7 @@ config X86
select HAVE_FUNCTION_ARG_ACCESS_API
select HAVE_STACKPROTECTOR if CC_HAS_SANE_STACKPROTECTOR
select HAVE_STACK_VALIDATION if X86_64
+ select HAVE_STATIC_CALL_OUTLINE
select HAVE_RSEQ
select HAVE_SYSCALL_TRACEPOINTS
select HAVE_UNSTABLE_SCHED_CLOCK
diff --git a/arch/x86/include/asm/static_call.h b/arch/x86/include/asm/static_call.h
new file mode 100644
index 000000000000..6e9ad5969ec2
--- /dev/null
+++ b/arch/x86/include/asm/static_call.h
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_STATIC_CALL_H
+#define _ASM_STATIC_CALL_H
+
+/*
+ * Manually construct a 5-byte direct JMP to prevent the assembler from
+ * optimizing it into a 2-byte JMP.
+ */
+#define __ARCH_STATIC_CALL_JMP_LABEL(key) ".L" __stringify(key ## _after_jmp)
+#define __ARCH_STATIC_CALL_TRAMP_JMP(key, func) \
+ ".byte 0xe9 \n" \
+ ".long " #func " - " __ARCH_STATIC_CALL_JMP_LABEL(key) "\n" \
+ __ARCH_STATIC_CALL_JMP_LABEL(key) ":"
+
+/*
+ * This is a permanent trampoline which does a direct jump to the function.
+ * The direct jump get patched by static_call_update().
+ */
+#define ARCH_DEFINE_STATIC_CALL_TRAMP(key, func) \
+ asm(".pushsection .text, \"ax\" \n" \
+ ".align 4 \n" \
+ ".globl " STATIC_CALL_TRAMP_STR(key) " \n" \
+ ".type " STATIC_CALL_TRAMP_STR(key) ", @function \n" \
+ STATIC_CALL_TRAMP_STR(key) ": \n" \
+ __ARCH_STATIC_CALL_TRAMP_JMP(key, func) " \n" \
+ ".popsection \n")
+
+#endif /* _ASM_STATIC_CALL_H */
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 8824d01c0c35..82acc8a28429 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -62,6 +62,7 @@ obj-y += tsc.o tsc_msr.o io_delay.o rtc.o
obj-y += pci-iommu_table.o
obj-y += resource.o
obj-y += irqflags.o
+obj-y += static_call.o

obj-y += process.o
obj-y += fpu/
diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
new file mode 100644
index 000000000000..8026d176f25c
--- /dev/null
+++ b/arch/x86/kernel/static_call.c
@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/static_call.h>
+#include <linux/memory.h>
+#include <linux/bug.h>
+#include <asm/text-patching.h>
+#include <asm/nospec-branch.h>
+
+#define CALL_INSN_SIZE 5
+
+void static_call_bp_handler(void);
+void *bp_handler_dest;
+
+asm(".pushsection .text, \"ax\" \n"
+ ".globl static_call_bp_handler \n"
+ ".type static_call_bp_handler, @function \n"
+ "static_call_bp_handler: \n"
+ "ANNOTATE_RETPOLINE_SAFE \n"
+ "jmp *bp_handler_dest \n"
+ ".popsection \n");
+
+void arch_static_call_transform(void *site, void *tramp, void *func)
+{
+ s32 dest_relative;
+ unsigned long insn;
+ unsigned char insn_opcode;
+ unsigned char opcodes[CALL_INSN_SIZE];
+
+ insn = (unsigned long)tramp;
+
+ mutex_lock(&text_mutex);
+
+ insn_opcode = *(unsigned char *)insn;
+ if (insn_opcode != 0xe8 && insn_opcode != 0xe9) {
+ WARN_ONCE(1, "unexpected static call insn opcode 0x%x at %pS",
+ insn_opcode, (void *)insn);
+ goto done;
+ }
+
+ dest_relative = (long)(func) - (long)(insn + CALL_INSN_SIZE);
+
+ opcodes[0] = insn_opcode;
+ memcpy(&opcodes[1], &dest_relative, CALL_INSN_SIZE - 1);
+
+ /* Set up the variable for the breakpoint handler: */
+ bp_handler_dest = func;
+
+ /* Patch the call site: */
+ text_poke_bp((void *)insn, opcodes, CALL_INSN_SIZE,
+ static_call_bp_handler);
+
+done:
+ mutex_unlock(&text_mutex);
+}
+EXPORT_SYMBOL_GPL(arch_static_call_transform);
diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index c8d0da1ef6b2..651f4d784377 100644
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h
@@ -149,7 +149,7 @@ struct static_call_key {
.func = _func, \
.tramp = STATIC_CALL_TRAMP(key), \
}; \
- ARCH_DEFINE_STATIC_CALL_TRAMP(key, func)
+ ARCH_DEFINE_STATIC_CALL_TRAMP(key, _func)

#define static_call(key, args...) STATIC_CALL_TRAMP(key)(args)

--
2.17.2
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