Re: [dpdk-dev] [PATCH v3] eal/arm64: fix rdtsc precise version

[Linhaifeng <haifeng.lin@huawei.com>]

> -----Original Message-----
> From: Jerin Jacob [mailto:jerinjacobk@gmail.com]
> Sent: Tuesday, March 10, 2020 6:47 PM
> To: Linhaifeng <haifeng.lin@huawei.com>
> Cc: Gavin Hu <Gavin.Hu@arm.com>; dev@dpdk.org; thomas@monjalon.net;
> chenchanghu <chenchanghu@huawei.com>; xudingke
> <xudingke@huawei.com>; Lilijun (Jerry) <jerry.lilijun@huawei.com>; Honnappa
> Nagarahalli <Honnappa.Nagarahalli@arm.com>; Steve Capper
> <Steve.Capper@arm.com>; nd <nd@arm.com>
> Subject: Re: [PATCH v3] eal/arm64: fix rdtsc precise version
> 
> On Tue, Mar 10, 2020 at 3:09 PM Linhaifeng <haifeng.lin@huawei.com> wrote:
> >
> > In order to get more accurate the cntvct_el0 reading, SW must invoke
> > isb and arch_counter_enforce_ordering.
> >
> > Reference of linux kernel:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tre
> > e/arch/arm64/include/asm/arch_timer.h?h=v5.5#n220
> >
> > Signed-off-by: Haifeng Lin <haifeng.lin@huawei.com>
> 
> Not addressed Fixes: comment
> http://mails.dpdk.org/archives/dev/2020-March/159547.html
> 
> > ---
> >  .../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()
> 
> Not addressed comment
> http://mails.dpdk.org/archives/dev/2020-March/159547.html
> 
> 
> > +
> >  /*------------------------ 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)
> > +
> 
> Not addressed the comments in
> http://mails.dpdk.org/archives/dev/2020-March/159547.html
> 
> Gavin, Linhaifeng,
> I don't think, this ordering is valid in the DPDK context.
> 
> See the patch from Will.
> https://lore.kernel.org/patchwork/patch/1076132/
> 
> Thoughts?

I think arch_counter_enforce_ordering maybe is invalid but isb must be valid for DPDK context.

> 
> 
> 
> >  static inline uint64_t
> >  rte_rdtsc_precise(void)
> >  {
> > -       rte_mb();
> > -       return rte_rdtsc();
> > +       uint64_t tsc;
> > +
> > +       rte_isb();
> > +       tsc = rte_rdtsc();
> 
> Not addressed the comments in
> http://mails.dpdk.org/archives/dev/2020-March/159547.html
> 
> > +       arch_counter_enforce_ordering(tsc);
> > +       return tsc;
> >  }
> >
> >  static inline uint64_t
> > --
> > 2.24.1.windows.2
> >