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

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Message-ID: <20160527101036.GC7865@e104818-lin.cambridge.arm.com>
Date:	Fri, 27 May 2016 11:10:37 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	David Miller <davem@...emloft.net>
Cc:	arnd@...db.de, ynorov@...iumnetworks.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-arch@...r.kernel.org,
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	schwidefsky@...ibm.com, heiko.carstens@...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 Thu, May 26, 2016 at 12:43:44PM -0700, David Miller wrote:
> From: Catalin Marinas <catalin.marinas@....com>
> Date: Thu, 26 May 2016 15:20:58 +0100
> 
> > We can solve (a) by adding more __SC_WRAP annotations in the generic
> > unistd.h.
>  ...
> 
> I really think it's much more robust to clear the tops of the registers
> by default.  Then you won't be auditing constantly and adding more and
> more wrappers.

I think we could avoid adding a new __SC_WRAP by redefining __SYSCALL
for ILP32 to always invoke a wrapper. But given the wrapper overhead,
cache locality, I don't think we would notice any performance difference
in either case.

> You can't even quantify the performance gains for me in any precise
> way.  Whatever you gain by avoiding the 64-bit
> decompostion/reconstitution for those few system calls with 64-bit
> registers, you are losing by calling the wrappers for more common
> system calls, more often.

I hope Yury can provide some numbers. All being equal, I would go for
the lowest code maintenance cost (which is probably less annotations and
wrappers).

> "it's more natural to pass 64-bit values in a register" is not a clear
> justification for this change.

It's more related to how we went about the ILP32 ABI. We initially asked
for a 64-bit native ABI similar to x32 until the libc-alpha community
raised some POSIX compliance issues on time structures. So we decided to
go for a 32-bit-like ABI while keeping the syscall interface close to
the AArch64/ILP32 procedure calling standard (64-bit values passed in a
single register). And now we have this discussion, revisiting this
decision again (which is perfectly fine, we better get it right before
any merging plans; thanks for your input).

-- 
Catalin