From: Jerin Jacob <jerin.jacob@caviumnetworks.com>
To: Ola Liljedahl <Ola.Liljedahl@arm.com>
Cc: Jan Viktorin <viktorin@rehivetech.com>,
"Gavin Hu (Arm Technology China)" <Gavin.Hu@arm.com>,
"dev@dpdk.org" <dev@dpdk.org>,
"thomas@monjalon.net" <thomas@monjalon.net>
Subject: Re: [dpdk-dev] [PATCH] eal/armv7: add support for rte pause
Date: Mon, 8 Oct 2018 11:57:29 +0530 [thread overview]
Message-ID: <20181008062728.GA12197@jerin> (raw)
In-Reply-To: <C059DF2C-08BD-47A1-98FF-362CAA609C99@arm.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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next prev parent reply other threads:[~2018-10-08 6:27 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-07 6:31 Jerin Jacob
2018-10-07 21:09 ` Ola Liljedahl
2018-10-08 6:27 ` Jerin Jacob [this message]
2018-10-08 8:25 ` Ola Liljedahl
2018-10-08 8:41 ` Jerin Jacob
2018-10-08 10:51 ` Ola Liljedahl
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