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

From: Oleg Nesterov
Date: Tue Jul 29 2014 - 15:23:33 EST

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

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,

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


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.


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