linux-kernel - Re: [PATCH v2] x86/syscall: make sys

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Message-ID: <20251220204059.GA272712@noisy.programming.kicks-ass.net>
Date: Sat, 20 Dec 2025 21:40:59 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Brian Gerst <brgerst@...il.com>
Cc: ellyndra <ellyesparza8@...il.com>, linux-kernel@...r.kernel.org,
	luto@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com
Subject: Re: [PATCH v2] x86/syscall: make sys_call helpers static

On Sat, Dec 20, 2025 at 09:32:27AM -0500, Brian Gerst wrote:

> > Ofcourse. But I'm not sure I understand your concern. Both
> > x{64,32}_sys_call() only have a single caller, inlining them avoids the
> > whole call, tail or otherwise, how is that a bad thing?
> >
> 
>    text       data        bss        dec        hex    filename
>   13194        128          0      13322       340a
> arch/x86/entry/syscall_64.o
>   10239        136          0      10375       2887
> arch/x86/entry/syscall_64.o.orig
> 
> That's a 3k increase in text size when it's inlined.

Oh, you're talking about the calls in the switch; when that's inlined it
ends up generating a pile of doo.

The below should do.

---
diff --git a/arch/x86/entry/syscall_32.c b/arch/x86/entry/syscall_32.c
index a67a644d0cfe..47ef0a10e42e 100644
--- a/arch/x86/entry/syscall_32.c
+++ b/arch/x86/entry/syscall_32.c
@@ -41,12 +41,14 @@ const sys_call_ptr_t sys_call_table[] = {
 #endif
 
 #define __SYSCALL(nr, sym) case nr: return __ia32_##sym(regs);
-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_begin();
 	switch (nr) {
 	#include <asm/syscalls_32.h>
 	default: return __ia32_sys_ni_syscall(regs);
 	}
+	instrumentation_end();
 }
 
 static __always_inline int syscall_32_enter(struct pt_regs *regs)
diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
index b6e68ea98b83..0f26f3f60b4a 100644
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -32,23 +32,33 @@ const sys_call_ptr_t sys_call_table[] = {
 #undef  __SYSCALL
 
 #define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs);
-long x64_sys_call(const struct pt_regs *regs, unsigned int nr)
+/*
+ * Explicitly noinline such that the whole switch table ends up generating
+ * tail-calls. When inlined into do_syscall_x64() it will 
+ */
+static noinstr long x64_sys_call(const struct pt_regs *regs, unsigned int nr)
 {
+	instrumentation_begin();
 	switch (nr) {
 	#include <asm/syscalls_64.h>
 	default: return __x64_sys_ni_syscall(regs);
 	}
+	instrumentation_end();
 }
 
-#ifdef CONFIG_X86_X32_ABI
-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)
 {
+	instrumentation_begin();
+#ifdef CONFIG_X86_X32_ABI
 	switch (nr) {
 	#include <asm/syscalls_x32.h>
 	default: return __x64_sys_ni_syscall(regs);
 	}
-}
+#else
+	return __x64_sys_ni_syscall(regs);
 #endif
+	instrumentation_end();
+}
 
 static __always_inline bool do_syscall_x64(struct pt_regs *regs, int nr)
 {
diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
index c10dbb74cd00..59a406074dc0 100644
--- a/arch/x86/include/asm/syscall.h
+++ b/arch/x86/include/asm/syscall.h
@@ -20,14 +20,6 @@
 typedef long (*sys_call_ptr_t)(const struct pt_regs *);
 extern const sys_call_ptr_t sys_call_table[];
 
-/*
- * These may not exist, but still put the prototypes in so we
- * can use IS_ENABLED().
- */
-extern long ia32_sys_call(const struct pt_regs *, unsigned int nr);
-extern long x32_sys_call(const struct pt_regs *, unsigned int nr);
-extern long x64_sys_call(const struct pt_regs *, unsigned int nr);
-
 /*
  * Only the low 32 bits of orig_ax are meaningful, so we return int.
  * This importantly ignores the high bits on 64-bit, so comparisons