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

Jianbo Liu jianbo.liu at linaro.org
Thu Jan 5 07:47:27 CET 2017


On 5 January 2017 at 14:24, Jerin Jacob <jerin.jacob at caviumnetworks.com> wrote:
> On Thu, Jan 05, 2017 at 01:31:44PM +0800, Jianbo Liu wrote:
>> 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?
>
> Adding David Marchand, EAL Maintainer.
>
> Instead of rte_io_?. I thought, IO specific constraints can be abstracted
> here in rte_io_*. Apart from arm, there other arch like "arc" has similar
> constraints. IMHO, no harm in keeping that abstraction.
>
> Thoughts ?
>
> http://lxr.free-electrons.com/ident?i=__iormb
>
>>
>> >
>> > 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?
>
> It is their for arm,arm64,arc architectures in Linux kernel. Please check writel
> implementation for arm64
>
> http://lxr.free-electrons.com/source/arch/arm64/include/asm/io.h#L143
>

Yes, I knew. But I'm afraid it will be mixed with dma_*mb by someone else.


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