seccomp: Document two-phase seccomp and arch-provided seccomp_data

The description of how archs should implement seccomp filters was
still strictly correct, but it failed to describe the newly
available optimizations.
Signed-off-by: NAndy Lutomirski <luto@amacapital.net>
Signed-off-by: NKees Cook <keescook@chromium.org>

arch/Kconfig

@@ -323,6 +323,17 @@ config HAVE_ARCH_SECCOMP_FILTER
 	    results in the system call being skipped immediately.
 	  - seccomp syscall wired up
 
+	  For best performance, an arch should use seccomp_phase1 and
+	  seccomp_phase2 directly.  It should call seccomp_phase1 for all
+	  syscalls if TIF_SECCOMP is set, but seccomp_phase1 does not
+	  need to be called from a ptrace-safe context.  It must then
+	  call seccomp_phase2 if seccomp_phase1 returns anything other
+	  than SECCOMP_PHASE1_OK or SECCOMP_PHASE1_SKIP.
+
+	  As an additional optimization, an arch may provide seccomp_data
+	  directly to seccomp_phase1; this avoids multiple calls
+	  to the syscall_xyz helpers for every syscall.
+
 config SECCOMP_FILTER
 	def_bool y
 	depends on HAVE_ARCH_SECCOMP_FILTER && SECCOMP && NET