Re: [dpdk-dev] [PATCH] eal/armv7: add support for rte pause

[Ola Liljedahl <Ola.Liljedahl@arm.com>]

On 07/10/2018, 08:32, "Jerin Jacob" <jerin.jacob@caviumnetworks.com> wrote:

    Add support for rte_pause() implementation for armv7.

    Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
    ---

    The reference implementation for Linux's cpu_relax() for armv7 is at
    https://elixir.bootlin.com/linux/latest/source/arch/arm/include/asm/processor.h#L100

    ---
     lib/librte_eal/common/include/arch/arm/rte_pause_32.h | 4 +++-
     1 file changed, 3 insertions(+), 1 deletion(-)

    diff --git a/lib/librte_eal/common/include/arch/arm/rte_pause_32.h b/lib/librte_eal/common/include/arch/arm/rte_pause_32.h
    index d4768c7a9..9b856e0cf 100644
    --- a/lib/librte_eal/common/include/arch/arm/rte_pause_32.h
    +++ b/lib/librte_eal/common/include/arch/arm/rte_pause_32.h
    @@ -9,11 +9,13 @@
     extern "C" {
     #endif

    -#include <rte_common.h>
    +#include <rte_atomic.h>
    +
     #include "generic/rte_pause.h"

     static inline void rte_pause(void)
     {
    +rte_compiler_barrier();
The compiler barrier is not mandated by the DPDK documentation for rte_pause():
http://doc.dpdk.org/api/rte__pause_8h.html

You have to go all the way to the source and GCC documentation to discover that for GCC, rte_pause calls _mm_pause() which in turn is implemented using __builtin_ia32_pause().
https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/X86-Built-in-Functions.html
void __builtin_ia32_pause (void)
Generates the pause machine instruction with a compiler memory barrier.


If you are using C11 atomic operations e.g. for polling a location, the atomic operations will be able to provide the required semantics (e.g. don't merge atomic loads from different iterations of a loop, optionally provide acquire and/or release (or stronger) ordering. A compiler barrier here interferes with the (possibly weaker) barriers from the atomic operations. We could use a C11-version of rte_pause() that doesn't have the compiler barrier. But actually, we want support for WFE, x86 also has something similar now, MONITOR/MWAIT?.

-- Ola


     }

     #ifdef __cplusplus
    --
    2.19.0



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