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Commit 1dcf74f6 authored by Andy Lutomirski's avatar Andy Lutomirski Committed by H. Peter Anvin
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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: default avatarAndy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/a3dbd267ee990110478d349f78cccfdac5497a84.1409954077.git.luto@amacapital.net


Signed-off-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
parent 54eea995

arch/x86/kernel/entry_64.S

+15 −23
Original line number Diff line number Diff line
@@ -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 $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