Re: 8aeb879baf12 - significant system call latency regression, bisected

[Peter Zijlstra]

On Sun, Jun 14, 2026 at 07:07:50PM -0700, H. Peter Anvin wrote:

> It uses the same hack as the Makefile to deal with function alignment with a
> prefix: it adds unnecessary NOPs so that the pre-alignment and
> post-alignment are the same. At the end of the day this really ought to be
> fixed in gcc.

And clang, but I don't think they can, it wrecks the 'ABI' they have in
place with the current set of arguments. Which I agree is somewhat
unfortunate, but it is what it is.

> diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
> index 71f032504e73..337e3e53d262 100644
> --- a/arch/x86/entry/syscall_64.c
> +++ b/arch/x86/entry/syscall_64.c
> @@ -9,6 +9,14 @@
>  #include <linux/nospec.h>
>  #include <asm/syscall.h>
>  
> +#ifdef CONFIG_CALL_PADDING
> +# define _pfe(x) __attribute((patchable_function_entry(x,x)))
> +#else
> +# define _pfe(x)
> +#endif
> +#define _align_func(x) __aligned(x) _pfe(x-CONFIG_FUNCTION_ALIGNMENT+CONFIG_FUNCTION_PADDING_BYTES)
> +#define align_func(x) _align_func((x) < CONFIG_FUNCTION_ALIGNMENT ? CONFIG_FUNCTION_ALIGNMENT : (x))
> +
>  #define __SYSCALL(nr, sym) extern long __x64_##sym(const struct pt_regs *);
>  #define __SYSCALL_NORETURN(nr, sym) extern long __noreturn __x64_##sym(const struct pt_regs *);
>  #include <asm/syscalls_64.h>
> @@ -32,7 +40,7 @@ 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)
> +long align_func(32) x64_sys_call(const struct pt_regs *regs, unsigned int nr)
>  {
>  	switch (nr) {
>  	#include <asm/syscalls_64.h>
> @@ -41,7 +49,7 @@ long x64_sys_call(const struct pt_regs *regs, unsigned int nr)
>  }
>  
>  #ifdef CONFIG_X86_X32_ABI
> -long x32_sys_call(const struct pt_regs *regs, unsigned int nr)
> +long align_func(32) x32_sys_call(const struct pt_regs *regs, unsigned int nr)
>  {
>  	switch (nr) {
>  	#include <asm/syscalls_x32.h>

This more or less works by accident, in general your align_func() macro
is horrendously broken when you consider kCFI. By changing the
patchable_function_entry attribute like this, the kCFI hash ends up at a
different location and things go side-ways really really fast.

The only reason it works here is that this function is never indirectly
called and so the kCFI ABI violation is immaterial.