linux-kernel - Re: [PATCH 1/2] perf tools: allow building for ARM

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Date:	Fri, 11 Dec 2009 12:26:57 +0100
From:	Mikael Pettersson <mikpe@...uu.se>
To:	Jamie Iles <jamie.iles@...ochip.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] perf tools: allow building for ARM

Jamie Iles writes:
 > On Fri, Dec 11, 2009 at 11:38:48AM +0100, Ingo Molnar wrote:
 > > 
 > > * Jamie Iles <jamie.iles@...ochip.com> wrote:
 > > 
 > > > On Fri, Dec 11, 2009 at 11:23:16AM +0100, Ingo Molnar wrote:
 > > > > cpu_relax() looks fine, but rmb() seems not to match the one that can be 
 > > > > found in arch/arm/:
 > > > > 
 > > > > arch/arm/include/asm/system.h:#define rmb()		dmb()
 > > > > arch/arm/include/asm/system.h:#define rmb()	do { if (arch_is_coherent()) dmb(); else barrier(); } while (0)
 > > > > arch/arm/include/asm/system.h:#define smp_rmb()	rmb()
 > > > > 
 > > > > arch/arm/include/asm/system.h:#define dmb() __asm__ __volatile__ ("dmb" : : : "memory")
 > > > > arch/arm/include/asm/system.h:#define dmb() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" \
 > > > > arch/arm/include/asm/system.h:#define dmb() __asm__ __volatile__ ("" : : : "memory")
 > > > > arch/arm/include/asm/system.h:#define dmb() __asm__ __volatile__ ("" : : : "memory")
 > > >
 > > > The implementation of the barriers depend on the CPU arch revision 
 > > > which is defined in the kernel config. As the perf tools don't use the 
 > > > kernel config, we don't know here what arch revision we're building 
 > > > for. Perhaps we need a LINUX_ARM_ARCH parameter when building for ARM 
 > > > so we can pick the correct one.
 > > 
 > > rmb() is used in two places in perf:
 > > 
 > >  tools/perf/builtin-record.c:    rmb();
 > >  tools/perf/builtin-top.c:       rmb();
 > > 
 > > to interact with the shared kernel/user ring-buffer. Getting a barrier 
 > > wrong there may cause hickups in recording.
 > > 
 > > Could you tell me a bit more about this ARM instruction - is the 'DMB' 
 > > instruction used on all SMP ARM cores? Can it be used unconditionally, 
 > > or is the instruction undefined on certain versions? To get the ball 
 > > rolling we could use it unconditionally in the initial patch, but this 
 > > needs to be solved i suspect.
 > There are a few cases we need to deal with:
 > 	- v7 SMP: DMB instruction
 > 	- v6 SMP: MCR coprocessor instruction
 > 	- v5 and earlier no instructions for barriers.
 > 
 > Looking at the TRM for a v7 core (cortex A9) the MCR instruction that v6 uses
 > is deprecated but still present. I suspect we could use this to cover the v6
 > and v7 cores but we wouldn't be able to do soft perf events on v5 or earlier
 > (which don't have hardware counters).

The correct solution is to invoke a kernel-exported CPU-specific helper
function in the ARM kernel helper page.

I see a __kuser_memory_barrier entry there which maps to smp_dmb.
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