Re: [PATCH v4 0/5] x86: two-phase syscall tracing and seccomp fastpath

From: Andy Lutomirski
Date: Tue Jul 29 2014 - 16:54:39 EST

On Jul 29, 2014 12:22 PM, "Oleg Nesterov" <oleg@xxxxxxxxxx> wrote:
> Andy, to avoid the confusion: I am not trying to review this changes.
> As you probably know my understanding of asm code in entry.S is very
> limited.
> Just a couple of questions to ensure I understand this correctly.
> On 07/28, Andy Lutomirski wrote:
> >
> > This is both a cleanup and a speedup. It reduces overhead due to
> > installing a trivial seccomp filter by 87%. The speedup comes from
> > avoiding the full syscall tracing mechanism for filters that don't
> And only after I look at 5/5 I _seem_ to actually understand where
> this speedup comes from.
> So. Currently tracesys: path always lead to "iret" after syscall, with
> this change we can avoid it if phase_1() returns zero, correct?
> And, this also removes the special TIF_SYSCALL_AUDIT-only case in entry.S,
> cool.
> I am wondering if we can do something similar with do_notify_resume() ?
> Stupid question. To simplify, lets forget that syscall_trace_enter()
> already returns the value. Can't we simplify the asm code if we do
> not export 2 functions, but make syscall_trace_enter() return
> "bool slow_path_is_needed". So that "tracesys:" could do
> // pseudo code
> tracesys:
> call syscall_trace_enter
> if (!slow_path_is_needed) {
> addq REST_SKIP, %rsp
> jmp system_call_fastpath
> }
> ...
> ?
> Once again, I am just curious, it is not that I actually suggest to consider
> this option.

We could, but this would lose a decent amount of the speedup. I could
try it and benchmark it, but I'm guessing that the save and restore is
kind of expensive. This will make audit slower than it currently is,
which may also annoy some people. (Not me.)

I'm also not convinced that it would be much simpler. My code is currently:

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 */

movq %rsp, %rdi
movq $AUDIT_ARCH_X86_64, %rsi
movq %rax,%rdx
call syscall_trace_enter_phase2


... slow path here ...

It would end up looking more like (totally untested):

mov %rsp, %rdi
movq $AUDIT_ARCH_X86_64, %rsi
call syscall_trace_enter
test [whatever condition]
j[cond] system_call_fastpath

... slow path here ...

So it's a bit simpler. Oddly, the ia32entry code doesn't have this
multiple syscall path distinction.

SAVE_REST is 6 movq instructions and a subq. FIXUP_TOP_OF_STACK is 7
movqs (and 8 if I ever get my way). RESTORE_TOP_OF_STACK is 4.
RESTORE_REST is 6 movqs and an adsq. So we're talking about avoiding
21 movqs, and addq, and a subq. That may be significant. (And I
suspect that the difference is much larger on platforms like arm64,
but that's a separate issue.)
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