linux-kernel - Re: [PATCH 1/3] MIPS: R6: Use lightweight SYNC instruction in smp

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Date:	Wed, 27 Jan 2016 21:28:49 -0500
From:	Joshua Kinard <kumba@...too.org>
To:	Ralf Baechle <ralf@...ux-mips.org>,
	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	macro@...ux-mips.org
Cc:	linux-mips@...ux-mips.org, benh@...nel.crashing.org,
	will.deacon@....com, linux-kernel@...r.kernel.org,
	markos.chandras@...tec.com, Steven.Hill@...tec.com,
	alexander.h.duyck@...hat.com, davem@...emloft.net
Subject: Re: [PATCH 1/3] MIPS: R6: Use lightweight SYNC instruction in smp_*
 memory barriers

On 06/05/2015 09:10, Ralf Baechle wrote:
> On Mon, Jun 01, 2015 at 05:09:34PM -0700, Leonid Yegoshin wrote:
> 
> Leonid,
> 
> to me the biggest technical problem with this patch is that the new Kconfig
> option is user visible.  This is the kind of deeply technical options
> which exceeds the technical knowledge of most users, so it should probably
> be driven by a select.
> 
> We probably also want to enquire how old CPUs from before the invention
> of the stype field are behaving.  If those as I hope for all treat an
> stype != 0 as stype 0 we could simply drop the option.  But we might
> simply be out of luck - dunno.
> 
> Maciej,
> 
> do you have an R4000 / R4600 / R5000 / R7000 / SiByte system at hand to
> test this?  I think we don't need to test that SYNC actually works as
> intended but the simpler test that SYNC <stype != 0> is not causing a
> illegal instruction exception is sufficient, that is if something like
> 
> int main(int argc, charg *argv[])
> {
> 	asm("	.set	mips2		\n"
> 	"	sync	0x10		\n"
> 	"	sync	0x13		\n"
> 	"	sync	0x04		\n"
> 	"	.set	mips 0		\n");
> 
> 	return 0;
> }
> 
> doesn't crash we should be ok.
> 
> The kernel's SYNC emulation should already be ok.  We ignore the stype
> field entirely and for a uniprocessor R2000/R3000 that should be just
> the right thing.
> 
>   Ralf

I tried just compiling this on my SGI O2, which has an RM7000 CPU, and it is
refusing to even compile (after fixing typos):

# gcc -O2 -pipe sync_test.c -o sync_test
{standard input}: Assembler messages:
{standard input}:19: Error: invalid operands `sync 0x10'
{standard input}:20: Error: invalid operands `sync 0x13'
{standard input}:21: Error: invalid operands `sync 0x04'

So a bit of searching landed me here:
http://stackoverflow.com/questions/3599564/gnu-assembler-for-mips-how-to-emit-sync-instructions

And I recoded the sync insns like this:

int main(int argc, char *argv[])
{
	__asm__ volatile (      \
	"	.set    mips2				\n"
	"	.word (0x0000000f | (0x10 << 6))	\n"
	"	.word (0x0000000f | (0x14 << 6))	\n"
	"	.word (0x0000000f | (0x04 << 6))	\n"
	"	.set    mips0				\n"
	);

	return 0;
}

And the program compiles successfully and executes with no noticeable oddities
or errors.  Nothing in dmesg, no crashes, booms, or disappearance of small
kittens.  I did a quick disassembly to make sure all three got emitted:

004005e0 <main>:
  4005e0:       27bdfff8        addiu   sp,sp,-8
  4005e4:       afbe0004        sw      s8,4(sp)
  4005e8:       03a0f021        move    s8,sp
  4005ec:       afc40008        sw      a0,8(s8)
  4005f0:       afc5000c        sw      a1,12(s8)
> 4005f4:       0000040f        sync.p
> 4005f8:       0000050f        0x50f
> 4005fc:       0000010f        0x10f
  400600:       00001021        move    v0,zero


Same effect on my Octane (IP30) w/ an R14000 CPU.  Tested inside a uclibc-based
chroot, but no change.  Executes successfully silently.

-- 
Joshua Kinard
Gentoo/MIPS
kumba@...too.org
6144R/F5C6C943 2015-04-27
177C 1972 1FB8 F254 BAD0 3E72 5C63 F4E3 F5C6 C943

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