Date:	Tue, 28 Jul 2015 17:02:06 +0100
From:	David Drysdale <drysdale@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Shuah Khan <shuahkh@....samsung.com>,
	Linux API <linux-api@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Jonathan Corbet <corbet@....net>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric B Munson <emunson@...mai.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Al Viro <viro@...iv.linux.org.uk>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Vivek Goyal <vgoyal@...hat.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	David Herrmann <dh.herrmann@...il.com>,
	"Theodore Ts'o" <tytso@....edu>, Kees Cook <keescook@...omium.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Milosz Tanski <milosz@...in.com>, Fam Zheng <famz@...hat.com>,
	Josh Triplett <josh@...htriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-doc@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 0/1] Document how to add a new syscall

On Tue, Jul 28, 2015 at 3:19 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Jul 28, 2015 at 07:59:16AM -0600, Shuah Khan wrote:
>> On 07/28/2015 05:41 AM, David Drysdale wrote:
>> > Given that I've gotten some of the details wrong in the past (and I've
>> > seen others do likewise), I thought it might be helpful to collate the
>> > best practices for adding a new system call to the kernel.
>> >
>> > Apologies for the wide circulation -- I've tried to include folk who've
>> > recently added or proposed a system call, as they're most likely to
>> > have opinions on:
>> >  - whether this a useful addition to Documentation/
>> >  - whether the details of the advice are correct and complete.
>> >
>> > Shuah, is there anything more that should be added for the Testing
>> > section in particular?
>> >
>>
>> David,
>>
>> Looks good. You could add a one liner on in some cases, it might be
>> necessary to install headers before test compiles. Other than that,
>> the information looks good.
>
> For perf we still use the syscall() wrapper and we have hardcoded
> (fallback) syscall numbers in there in case they've not yet reached
> unistd.h.
>
> People update their kernels far more often than their userspace headers.

How about:

diff --git a/Documentation/adding-syscalls.txt
b/Documentation/adding-syscalls.txt
index 5f52edda8951..b274c3d01edb 100644
--- a/Documentation/adding-syscalls.txt
+++ b/Documentation/adding-syscalls.txt
@@ -389,6 +389,11 @@ reviewers with a demonstration of how user space
programs will use the system
 call.  A good way to combine these aims is to include a simple self-test
 program in a new directory under tools/testing/selftests/.

+For a new system call, there will obviously be no libc wrapper function and so
+the test will need to invoke it using syscall(); also, if the system call
+involves a new userspace-visible structure, the corresponding header will need
+to be installed to compile the test.
+
 Make sure the selftest runs successfully on all supported architectures.  For
 example, check that it works when compiled as an x86_64 (-m64), x86_32 (-m32)
 and x32 (-mx32) ABI program.
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