[dpdk-dev] [PATCH v2 09/29] eal/arm64: define I/O device memory barriers for arm64

[Jianbo Liu]

On 4 January 2017 at 18:01, Jerin Jacob <jerin.jacob at caviumnetworks.com> wrote:
> On Tue, Jan 03, 2017 at 03:48:32PM +0800, Jianbo Liu wrote:
>> On 27 December 2016 at 17:49, Jerin Jacob
>> <jerin.jacob at caviumnetworks.com> wrote:
>> > CC: Jianbo Liu <jianbo.liu at linaro.org>
>> > Signed-off-by: Jerin Jacob <jerin.jacob at caviumnetworks.com>
>> > ---
>> >  lib/librte_eal/common/include/arch/arm/rte_atomic_64.h | 6 ++++++
>> >  1 file changed, 6 insertions(+)
>> >
>> > 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 78ebea2..ef0efc7 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
>> > @@ -88,6 +88,12 @@ static inline void rte_rmb(void)
>> >
>> >  #define rte_smp_rmb() dmb(ishld)
>> >
>> > +#define rte_io_mb() rte_mb()
>> > +
>> > +#define rte_io_wmb() rte_wmb()
>> > +
>> > +#define rte_io_rmb() rte_rmb()
>> > +
>>
>> I think it's better to use outer shareable dmb for io barrier, instead of dsb.
>
> Its is difficult to generalize. AFAIK, from the IO barrier perspective
> dsb would be the right candidate. But just for the DMA barrier between IO may
> be outer sharable dmb is enough. In-terms of performance implication, the
> fastpath code(door bell write) has been changed to relaxed write in all
> the drivers in this patchset and rte_io_* will be only
> used by rte_[read/write]8/16/32/64 which will be in slow-path.
> So, IMO, it better stick with dsb and its safe from the complete IO barrier
> perspective.

If so, why not use *mb() directly?

>
> At least on ThunderX, I couldn't see any performance difference between
> using dsb(st) and dmb(oshst) for dma write barrier before the doorbell register
> write in fastpath. In case there are platforms which has such performance difference,
> may be could add rte_dma_wmb() and rte_dma_rmb() in future like Linux kernel
> dma_wmb() and dma_rmb().(But i couldn't  see all the driver are using it,
> though)
>

But there is no io_*mb() in the kernel, so you want to be different?