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

From: Oleg Nesterov
Date: Wed Jul 30 2014 - 11:35:25 EST

On 07/29, Andy Lutomirski wrote:
> > 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.)

OK, thanks. We could probably simplify the logic in phase1 + phase2 if
it was a single function though.

> To put some more options on the table: there's an argument to be made
> that the whole fast-path/slow-path split isn't worth it. We could
> unconditionally set up a full frame for all syscalls. This means:

Or, at least, can't we allocate the full frame and avoid "add/sub %rsp"?

> This means:
> On the
> other hand, there's zero chance that this would be ready for 3.17.
> I'd tend to advocate for keeping the approach in my patches for now.

Yes, sure, I didn't try to convince you to change this code. Thanks.


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