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

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Message-ID: <20140730153259.GA25478@redhat.com>
Date:	Wed, 30 Jul 2014 17:32:59 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	linux-arch <linux-arch@...r.kernel.org>, X86 ML <x86@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LSM List <linux-security-module@...r.kernel.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 0/5] x86: two-phase syscall tracing and seccomp
	fastpath

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.

Oleg.

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