linux-kernel - Re: [PATCH 01/23] all: syscall wrappers: add documentation

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Message-ID: <20160527173653.GA19961@e104818-lin.cambridge.arm.com>
Date:	Fri, 27 May 2016 18:36:53 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Yury Norov <ynorov@...iumnetworks.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	David Miller <davem@...emloft.net>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-s390@...r.kernel.org, libc-alpha@...rceware.org,
	schwidefsky@...ibm.com, pinskia@...il.com, broonie@...nel.org,
	joseph@...esourcery.com, christoph.muellner@...obroma-systems.com,
	bamvor.zhangjian@...wei.com, szabolcs.nagy@....com,
	klimov.linux@...il.com, Nathan_Lynch@...tor.com, agraf@...e.de,
	Prasun.Kapoor@...iumnetworks.com, kilobyte@...band.pl,
	geert@...ux-m68k.org, philipp.tomsich@...obroma-systems.com
Subject: Re: [PATCH 01/23] all: syscall wrappers: add documentation

On Fri, May 27, 2016 at 07:58:06PM +0300, Yury Norov wrote:
> So, we have 3 options for now:
> 1. Clear top halves in entry.S which means we pass off_t as a pair.
>    The cost is performance (didn't measure it yet and doubt about it
>    makes serious impact). The advantage is simplicity and unification with
>    aarch32, as I mentioned above. And David likes it. And it mininizes
>    the amount of changes on glibc side.
> 2. Clear top halves in in separated file hosted wrappers.
> 3. Clear top halves in I-cache and tail optimization friendly in-site wrappers.
> 
> 2 and 3 are the same from ABI point of view.
> 
> 2 is the worst for me as it is the most complex in implementation and 
> I-cache and tail optimization non-friendly. But Heiko likes it.
>  
> 3 is what Catalin is talking about, and it was my initial approach.
> Though I didn't made compiler to do tail optimization, I think we can
> do it.

I don't fully understand the difference between 2 and 3. My comment was
more around annotating the wrappers in (2) with __naked to no longer
generate function prologue/epilogue. They would still be in a separate
kernel/compat_wrapper.c file.

I can't figure out how you would have in-place wrappers for all
syscalls. You can indeed handle the current COMPAT_SYSCALL_DEFINE via
__SC_DELOUSE (and penalising the AArch32/compat support slightly) but
there is no solution for native SYSCALL_DEFINE functions to do it
in-place.

> But 2 is what we have now. And I'd choose it. We'll never get ilp32 done
> if will roll back previously agreed decisions again and again.

I would rather roll back a decision than going ahead with a wrong one.
Note that this is *ABI*, not a driver that you can fix upstream later.

Since yesterday, I realised that (2) requires further annotations and
wrapping for the native and compat syscalls used by ILP32 just to cope
with pointers. Also given davem's comments, (1) starts to look a bit
more appealing (I don't like reverting such decisions either, I'd have
to review the code again and again).

-- 
Catalin