Re: [PATCH 10/10] powerpc: remove address space overrides using set_fs()

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>, Al Viro <viro@zeniv.linux.org.uk>,
	Michael Ellerman <mpe@ellerman.id.au>,
	the arch/x86 maintainers <x86@kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Kees Cook <keescook@chromium.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 10/10] powerpc: remove address space overrides using set_fs()
Date: Thu, 3 Sep 2020 09:20:35 +0200	[thread overview]
Message-ID: <6a6ea160-a661-4a15-777c-e26a487829d4@csgroup.eu> (raw)
In-Reply-To: <CAHk-=wiDCcxuHgENo3UtdFi2QW9B7yXvNpG5CtF=A6bc6PTTgA@mail.gmail.com>

Le 02/09/2020 à 20:02, Linus Torvalds a écrit :
> On Wed, Sep 2, 2020 at 8:17 AM Christophe Leroy
> <christophe.leroy@csgroup.eu> wrote:
>>
>>
>> With this fix, I get
>>
>> root@vgoippro:~# time dd if=/dev/zero of=/dev/null count=1M
>> 536870912 bytes (512.0MB) copied, 6.776327 seconds, 75.6MB/s
>>
>> That's still far from the 91.7MB/s I get with 5.9-rc2, but better than
>> the 65.8MB/s I got yesterday with your series. Still some way to go thought.
> 
> I don't see why this change would make any difference.
> 

Neither do I.

Looks like nowadays, CONFIG_STACKPROTECTOR has become a default.
I rebuilt the kernel without it, I now get a throughput of 99.8MB/s both 
without and with this series.

Looking at the generated code (GCC 10.1), a small change in a function 
seems to make large changes in the generated code when 
CONFIG_STACKPROTECTOR is set.

In addition to that, trivial functions which don't use the stack at all 
get a stack frame anyway when CONFIG_STACKPROTECTOR is set, allthough 
that's only -fstack-protector-strong. And there is no canary check.

Without CONFIG_STACKPROTECTOR:

c01572a0 <no_llseek>:
c01572a0:	38 60 ff ff 	li      r3,-1
c01572a4:	38 80 ff e3 	li      r4,-29
c01572a8:	4e 80 00 20 	blr

With CONFIG_STACKPROTECTOR (regardless of CONFIG_STACKPROTECTOR_STRONG 
or not):

c0164e08 <no_llseek>:
c0164e08:	94 21 ff f0 	stwu    r1,-16(r1)
c0164e0c:	38 60 ff ff 	li      r3,-1
c0164e10:	38 80 ff e3 	li      r4,-29
c0164e14:	38 21 00 10 	addi    r1,r1,16
c0164e18:	4e 80 00 20 	blr

Wondering why CONFIG_STACKPROTECTOR has become the default. It seems to 
imply a 10% performance loss even in the best case (91.7MB/s versus 
99.8MB/s)

Note that without CONFIG_STACKPROTECTOR_STRONG, I'm at 99.3MB/s, so 
that's really the _STRONG alternative that hurts.

Christophe