Re: [dpdk-dev] [PATCH v3 1/3] ring: read tail using atomic load

[Ola Liljedahl]

On 07/10/2018, 06:03, "Jerin Jacob" <jerin.ja...@caviumnetworks.com> wrote:

    How about fixing rte_pause() then?
    Meaning issuing power saving instructions on missing archs.
Rte_pause() implemented as NOP or YIELD on ARM will likely not save any power. 
You should use WFE for that.
I use this portable pattern:

        //Wait for our turn to signal consumers (producers) 
        if (UNLIKELY(__atomic_load_n(loc, __ATOMIC_RELAXED) != idx))
        {
            SEVL();
            while (WFE() && LDXR32(loc, __ATOMIC_RELAXED) != idx)
            {   
                DOZE();
            }
        }

    
For AArch64 with WFE usage enabled:
#define SEVL() sevl()
#define WFE() wfe()
#define LDXR32(a, b)  ldx32((a), (b))
#define DOZE() (void)0
static inline void sevl(void)
{
    __asm__ volatile("sevl" : : : );
}
static inline int wfe(void)
{
    __asm__ volatile("wfe" : : : "memory");
    return 1;
}

For architectures without WFE support:
#define SEVL() (void)0
#define WFE() 1
#define LDXR32(a, b) __atomic_load_n((a), (b))
#define DOZE() doze()
static inline void doze(void)
{
    __asm__ volatile("rep; nop" : : : );
}

-- Ola