x86/syscalls: Document the fact that syscalls 512-547 are a legacy mistake

Since this commit:

  6365b842aae4 ("x86/syscalls: Split the x32 syscalls into their own table")

there is no need for special x32-specific syscall numbers.  I forgot to
update the comments in syscall_64.tbl.  Add comments to make it clear to
future contributors that this range is a legacy wart.

Reported-by: Jessica Clarke <jrtc27@jrtc27.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/6c56fb4ddd18fc60a238eb4d867e4b3d97c6351e.1602471055.git.luto@kernel.org

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index f30d6ae9a6883c510eb07f821675b460a3fd8754..4adb5d2a3319e66f0e5ea7fe32b307a9ead3d97d 100644 (file)
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -363,10 +363,10 @@
 439    common  faccessat2              sys_faccessat2
 
 #
-# x32-specific system call numbers start at 512 to avoid cache impact
-# for native 64-bit operation. The __x32_compat_sys stubs are created
-# on-the-fly for compat_sys_*() compatibility system calls if X86_X32
-# is defined.
+# Due to a historical design error, certain syscalls are numbered differently
+# in x32 as compared to native x86_64.  These syscalls have numbers 512-547.
+# Do not add new syscalls to this range.  Numbers 548 and above are available
+# for non-x32 use.
 #
 512    x32     rt_sigaction            compat_sys_rt_sigaction
 513    x32     rt_sigreturn            compat_sys_x32_rt_sigreturn
@@ -404,3 +404,5 @@
 545    x32     execveat                compat_sys_execveat
 546    x32     preadv2                 compat_sys_preadv64v2
 547    x32     pwritev2                compat_sys_pwritev64v2
+# This is the end of the legacy x32 range.  Numbers 548 and above are
+# not special and are not to be used for x32-specific syscalls.