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.net
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 0bd6d3c..df088bb 100644 (file)
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -478,22 +478,6 @@ sysret_signal:
 
 #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
         * masked off.
@@ -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