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

[Jerin Jacob <jerin.jacob@caviumnetworks.com>]

-----Original Message-----
> Date: Sun, 7 Oct 2018 21:09:25 +0000
> From: Ola Liljedahl <Ola.Liljedahl@arm.com>
> To: Jerin Jacob <jerin.jacob@caviumnetworks.com>, Jan Viktorin
>  <viktorin@rehivetech.com>, "Gavin Hu (Arm Technology China)"
>  <Gavin.Hu@arm.com>
> CC: "dev@dpdk.org" <dev@dpdk.org>, "thomas@monjalon.net"
>  <thomas@monjalon.net>
> Subject: Re: [dpdk-dev] [PATCH] eal/armv7: add support for rte pause
> user-agent: Microsoft-MacOutlook/10.11.0.180909
> 
> External Email
> 
> 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

We can add that explicitly if required to inline with other arch. Just like
Linux kernel's cpu_relax()

> 
> 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.

Yes. IMO, it makes sense to have compiler memory barrier to make sure it
waits semantically at least WRT current rte_pause() usage.

> 
> 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

If it is WFE then who will wake up from the power saving state. SEV from the
other thread?

What would be a C11 version of rte_pause()?

> 
> -- Ola
> 
> 
>      }
> 
>      #ifdef __cplusplus
>     --
>     2.19.0
> 
> 
> 
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