[PATCH 3/6] x86_64/entry: Make syscall dispatch noinstr

[Peter Zijlstra <peterz@infradead.org>]

Being able to put breakpoints and the like inside syscall dispatch is
'unfortunate'. Simply push noinstr out one more layer.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/entry/syscall_32.c |    3 ++-
 arch/x86/entry/syscall_64.c |   13 +++++++++++--
 2 files changed, 13 insertions(+), 3 deletions(-)

--- a/arch/x86/entry/syscall_32.c
+++ b/arch/x86/entry/syscall_32.c
@@ -43,8 +43,9 @@ const sys_call_ptr_t sys_call_table[] =
 #define __SYSCALL(nr, sym) case nr: return __ia32_##sym(regs);
 
 /* The unsigned int @nr argument is intentional as it creates denser code in a 64-bit build */
-static noinline long ia32_sys_call(const struct pt_regs *regs, unsigned int nr)
+static noinstr long ia32_sys_call(const struct pt_regs *regs, unsigned int nr)
 {
+	instrumentation_weak();
 	switch (nr) {
 	#include <asm/syscalls_32.h>
 	default: return __ia32_sys_ni_syscall(regs);
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -34,17 +34,26 @@ const sys_call_ptr_t sys_call_table[] =
 #define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs);
 
 /* The unsigned int @nr argument is intentional as it creates denser code */
-static noinline long x64_sys_call(const struct pt_regs *regs, unsigned int nr)
+static noinstr long x64_sys_call(const struct pt_regs *regs, unsigned int nr)
 {
+	/*
+	 * Because -fno-jump-tables, this compiles into a binary branch tree
+	 * rather than a jump-table. As such @nr is not used as an array
+	 * index. Additionally, this is an out-of-line function on purpose,
+	 * such that all the actual syscall function calls are tail-calls,
+	 * returning to our caller for the common bits.
+	 */
+	instrumentation_weak();
 	switch (nr) {
 	#include <asm/syscalls_64.h>
 	default: return __x64_sys_ni_syscall(regs);
 	}
 }
 
-static noinline long x32_sys_call(const struct pt_regs *regs, unsigned int nr)
+static noinstr long x32_sys_call(const struct pt_regs *regs, unsigned int nr)
 {
 #ifdef CONFIG_X86_X32_ABI
+	instrumentation_weak();
 	switch (nr) {
 	#include <asm/syscalls_x32.h>
 	default: return __x64_sys_ni_syscall(regs);