Re: [dpdk-dev] [PATCH v2] cycles: add isb before read cntvct_el0

[Jerin Jacob <jerinjacobk@gmail.com>]

On Tue, Mar 10, 2020 at 1:39 PM Linhaifeng <haifeng.lin@huawei.com> wrote:

Please change the subject to  more meaningful one: Something like,

eal/arm64: fix rdtsc precise version

> We should use isb rather than dsb to sync system counter to cntvct_el0.

Please tell "why" in commit message, something like this

In order to get more accurate the cntvct_el0 reading, SW must invoke  .....

Add Fixes: as well.

# Make sure it is passing ./devtools/checkpatches.sh and
./devtools/check-git-log.sh

>
> Reference of linux kernel:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/include/asm/arch_timer.h?h=v5.5#n220
>
> Signed-off-by: Haifeng Lin <haifeng.lin@huawei.com>
> ---
>  .../common/include/arch/arm/rte_atomic_64.h   |  3 +++
>  .../common/include/arch/arm/rte_cycles_64.h   | 20 +++++++++++++++++--
>  2 files changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/lib/librte_eal/common/include/arch/arm/rte_atomic_64.h b/lib/librte_eal/common/include/arch/arm/rte_atomic_64.h
> index 859ae129d..2587f98a2 100644
> --- a/lib/librte_eal/common/include/arch/arm/rte_atomic_64.h
> +++ b/lib/librte_eal/common/include/arch/arm/rte_atomic_64.h
> @@ -21,6 +21,7 @@ extern "C" {
>
>  #define dsb(opt) asm volatile("dsb " #opt : : : "memory")
>  #define dmb(opt) asm volatile("dmb " #opt : : : "memory")
> +#define isb()    (asm volatile("isb" : : : "memory"))
>
>  #define rte_mb() dsb(sy)
>
> @@ -44,6 +45,8 @@ extern "C" {
>
>  #define rte_cio_rmb() dmb(oshld)
>
> +#define rte_isb() isb()

As David said, Don't add rte_ public symbols from header files.

> +
>  /*------------------------ 128 bit atomic operations -------------------------*/
>
>  #if defined(__ARM_FEATURE_ATOMICS) || defined(RTE_ARM_FEATURE_ATOMICS)
> diff --git a/lib/librte_eal/common/include/arch/arm/rte_cycles_64.h b/lib/librte_eal/common/include/arch/arm/rte_cycles_64.h
> index 68e7c7338..bc4e3f8e6 100644
> --- a/lib/librte_eal/common/include/arch/arm/rte_cycles_64.h
> +++ b/lib/librte_eal/common/include/arch/arm/rte_cycles_64.h
> @@ -18,6 +18,7 @@ extern "C" {
>   *   The time base for this lcore.
>   */
>  #ifndef RTE_ARM_EAL_RDTSC_USE_PMU
> +
>  /**
>   * This call is portable to any ARMv8 architecture, however, typically
>   * cntvct_el0 runs at <= 100MHz and it may be imprecise for some tasks.
> @@ -59,11 +60,26 @@ rte_rdtsc(void)
>  }
>  #endif
>
> +#define arch_counter_enforce_ordering(val) do {                                                \
> +       uint64_t tmp, _val = (val);                                                                             \
> +                                                                                                                                       \
> +       asm volatile(                                                                                                   \
> +       "       eor  %0, %1, %1\n"                                                                                      \
> +       "       add  %0, sp, %0\n"                                                                                      \
> +       "       ldr  xzr, [%0]"                                                                                         \
> +       : "=r" (tmp) : "r" (_val));                                                                             \
> +} while (0)

Please use static  inline function and change the name to
__rte_arm64_cntvct_el0_enforce_ordering() or so

> +
>  static inline uint64_t
>  rte_rdtsc_precise(void)
>  {
> -       rte_mb();
> -       return rte_rdtsc();
> +       uint64_t tsc;
> +
> +       rte_isb();
No need for new marco, If you think, it is needed make it as
__rte_arm64_isb() or so

> +       tsc = rte_rdtsc();
> +       arch_counter_enforce_ordering(tsc);
> +       return tsc;
>  }
>
>  static inline uint64_t
> --
> 2.24.1.windows.2