x86_64, entry: Use split-phase syscall_trace_enter for 64-bit syscalls

On KVM on my box, this reduces the overhead from an always-accept
seccomp filter from ~130ns to ~17ns.  Most of that comes from
avoiding IRET on every syscall when seccomp is enabled.

In extremely approximate hacked-up benchmarking, just bypassing IRET
saves about 80ns, so there's another 43ns of savings here from
simplifying the seccomp path.

The diffstat is also rather nice :)
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/a3dbd267ee990110478d349f78cccfdac5497a84.1409954077.git.luto@amacapital.netSigned-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/entry_64.S

@@ -477,22 +477,6 @@ sysret_signal:
 	jmp int_check_syscall_exit_work
 
 #ifdef CONFIG_AUDITSYSCALL
-	/*
-	 * Fast path for syscall audit without full syscall trace.
-	 * We just call __audit_syscall_entry() directly, and then
-	 * jump back to the normal fast path.
-	 */
-auditsys:
-	movq %r10,%r9			/* 6th arg: 4th syscall arg */
-	movq %rdx,%r8			/* 5th arg: 3rd syscall arg */
-	movq %rsi,%rcx			/* 4th arg: 2nd syscall arg */
-	movq %rdi,%rdx			/* 3rd arg: 1st syscall arg */
-	movq %rax,%rsi			/* 2nd arg: syscall number */
-	movl $AUDIT_ARCH_X86_64,%edi	/* 1st arg: audit arch */
-	call __audit_syscall_entry
-	LOAD_ARGS 0		/* reload call-clobbered registers */
-	jmp system_call_fastpath
-
 	/*
 	 * Return fast path for syscall audit.  Call __audit_syscall_exit()
 	 * directly and then jump back to the fast path with TIF_SYSCALL_AUDIT
@@ -510,17 +494,25 @@ sysret_audit:
 
 	/* Do syscall tracing */
 tracesys:
-#ifdef CONFIG_AUDITSYSCALL
-	testl $(_TIF_WORK_SYSCALL_ENTRY & ~_TIF_SYSCALL_AUDIT),TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
-	jz auditsys
-#endif
+	leaq -REST_SKIP(%rsp), %rdi
+	movq $AUDIT_ARCH_X86_64, %rsi
+	call syscall_trace_enter_phase1
+	test %rax, %rax
+	jnz tracesys_phase2		/* if needed, run the slow path */
+	LOAD_ARGS 0			/* else restore clobbered regs */
+	jmp system_call_fastpath	/*      and return to the fast path */
+
+tracesys_phase2:
 	SAVE_REST
 	FIXUP_TOP_OF_STACK %rdi
-	movq %rsp,%rdi
-	call syscall_trace_enter
+	movq %rsp, %rdi
+	movq $AUDIT_ARCH_X86_64, %rsi
+	movq %rax,%rdx
+	call syscall_trace_enter_phase2
+
 	/*
 	 * Reload arg registers from stack in case ptrace changed them.
-	 * We don't reload %rax because syscall_trace_enter() returned
+	 * We don't reload %rax because syscall_trace_entry_phase2() returned
 	 * the value it wants us to use in the table lookup.
 	 */
 	LOAD_ARGS ARGOFFSET, 1