linux-kernel - Re: [PATCH v4 RESEND] x86/asm/entry/32, selftests: Add 'test_syscall

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Message-ID: <CALCETrVWtxwssEeeDD8=Uw40AeUZ--yi2ejjcq+z=iCm5po+xA@mail.gmail.com>
Date:	Mon, 14 Sep 2015 10:54:24 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Will Drewry <wad@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v4 RESEND] x86/asm/entry/32, selftests: Add
 'test_syscall_vdso' test

On Sep 14, 2015 1:26 AM, "Ingo Molnar" <mingo@...nel.org> wrote:
>
>
> * Andy Lutomirski <luto@...capital.net> wrote:
>
> > > (I'm not very comfortable about additional six push/pops
> > > which are necessary for this to happen. I'm surprised
> > > maintainers tentatively agreed to that -
> > > I was grilled and asked to prove with measurements
> > > that *one* additional push+pop wasn't adding significant overhead).
> >
> > I suspect that I need to make the series faster.
> >
> > Also, int $0x80 isn't a fast path for any legitimate use case except
> > Debian, and I would argue that Debian is just buggy.
>
> So buggy in the sense of not making use of faster syscalls, right? It won't break
> in any visible way, correct?

Correct.  The only visible breakage I know of is that my unwind
selftest fails if compiled by a Debian toolchain, and I'll fix that in
v2.

>
> So if this heavy int80 syscall use happens even with the latest version of Debian
> as well then it would be nice to figure out what's wrong there, and provide an
> optimization patch to their libc guys or so - to make sure we fully understand the
> problem.

I looked for a bit and drew a blank, but I know I'm bad at
understanding Debian packaging and even worse at understanding the
mess that is glibc's pile of multiply compiled files.  I'll file a bug
or something.

--Andy
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