[dpdk-dev] [PATCH v2] eal: adjust barriers for IO on Armv8-a

[Honnappa Nagarahalli]

Hi Jerin,
	You had a comment earlier about deprecating rte_cio_[rw]mb. Let me know if you are ok with this patch and I can add those changes (replace references to rte_cio_[rw]mb with rte_io_[rw]mb and a deprecation notice).

Thanks,
Honnappa

> -----Original Message-----
> From: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
> Sent: Saturday, June 27, 2020 2:12 PM
> To: dev at dpdk.org; Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>;
> Ruifeng Wang <Ruifeng.Wang at arm.com>; jerinj at marvell.com;
> hemant.agrawal at nxp.com; Ajit Khaparde (ajit.khaparde at broadcom.com)
> <ajit.khaparde at broadcom.com>; igorch at amazon.com;
> thomas at monjalon.net; viacheslavo at mellanox.com;
> arybchenko at solarflare.com; bruce.richardson at intel.com
> Cc: nd <nd at arm.com>
> Subject: [PATCH v2] eal: adjust barriers for IO on Armv8-a
> 
> Change the barrier APIs for IO to reflect that Armv8-a is other-multi-copy
> atomicity memory model.
> 
> Armv8-a memory model has been strengthened to require other-multi-copy
> atomicity. This property requires memory accesses from an observer to
> become visible to all other observers simultaneously [3]. This means
> 
> a) A write arriving at an endpoint shared between multiple CPUs is
>    visible to all CPUs
> b) A write that is visible to all CPUs is also visible to all other
>    observers in the shareability domain
> 
> This allows for using cheaper DMB instructions in the place of DSB for devices
> that are visible to all CPUs (i.e. devices that DPDK caters to).
> 
> Please refer to [1], [2] and [3] for more information.
> 
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id
> =22ec71615d824f4f11d38d0e55a88d8956b7e45f
> [2] https://www.youtube.com/watch?v=i6DayghhA8Q
> [3] https://www.cl.cam.ac.uk/~pes20/armv8-mca/
> 
> Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
> Tested-by: Ruifeng Wang <ruifeng.wang at arm.com>
> ---
>  lib/librte_eal/arm/include/rte_atomic_64.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/lib/librte_eal/arm/include/rte_atomic_64.h
> b/lib/librte_eal/arm/include/rte_atomic_64.h
> index 7b7099cdc..e42f69edc 100644
> --- a/lib/librte_eal/arm/include/rte_atomic_64.h
> +++ b/lib/librte_eal/arm/include/rte_atomic_64.h
> @@ -1,6 +1,6 @@
>  /* SPDX-License-Identifier: BSD-3-Clause
>   * Copyright(c) 2015 Cavium, Inc
> - * Copyright(c) 2019 Arm Limited
> + * Copyright(c) 2020 Arm Limited
>   */
> 
>  #ifndef _RTE_ATOMIC_ARM64_H_
> @@ -19,11 +19,11 @@ extern "C" {
>  #include <rte_compat.h>
>  #include <rte_debug.h>
> 
> -#define rte_mb() asm volatile("dsb sy" : : : "memory")
> +#define rte_mb() asm volatile("dmb osh" : : : "memory")
> 
> -#define rte_wmb() asm volatile("dsb st" : : : "memory")
> +#define rte_wmb() asm volatile("dmb oshst" : : : "memory")
> 
> -#define rte_rmb() asm volatile("dsb ld" : : : "memory")
> +#define rte_rmb() asm volatile("dmb oshld" : : : "memory")
> 
>  #define rte_smp_mb() asm volatile("dmb ish" : : : "memory")
> 
> @@ -37,9 +37,9 @@ extern "C" {
> 
>  #define rte_io_rmb() rte_rmb()
> 
> -#define rte_cio_wmb() asm volatile("dmb oshst" : : : "memory")
> +#define rte_cio_wmb() rte_wmb()
> 
> -#define rte_cio_rmb() asm volatile("dmb oshld" : : : "memory")
> +#define rte_cio_rmb() rte_rmb()
> 
>  /*------------------------ 128 bit atomic operations -------------------------*/
> 
> --
> 2.17.1