RE: [EXT] Re: [PATCH] common/cnxk: use cas with release semantics for batch alloc

[Ruifeng Wang <Ruifeng.Wang@arm.com>]

> -----Original Message-----
> From: Ferruh Yigit <ferruh.yigit@intel.com>
> Sent: Tuesday, January 11, 2022 9:46 PM
> To: Ashwin Sekhar Thalakalath Kottilveetil <asekhar@marvell.com>;
> dev@dpdk.org; Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> Ruifeng Wang <Ruifeng.Wang@arm.com>
> Cc: Nithin Kumar Dabilpuram <ndabilpuram@marvell.com>;
> jerinj@marvell.com; Sunil Kumar Kori <skori@marvell.com>; Satha
> Koteswara Rao Kottidi <skoteshwar@marvell.com>; Pavan Nikhilesh
> Bhagavatula <pbhagavatula@marvell.com>; Kiran Kumar Kokkilagadda
> <kirankumark@marvell.com>; Satheesh Paul <psatheesh@marvell.com>;
> Anoob Joseph <anoobj@marvell.com>; Akhil Goyal <gakhil@marvell.com>
> Subject: Re: [EXT] Re: [PATCH] common/cnxk: use cas with release semantics
> for batch alloc
> 
> On 1/11/2022 12:26 PM, Ashwin Sekhar Thalakalath Kottilveetil wrote:
> > CAS is compare and swap. CASL is compare and swap with release
> semantics.
> >
> 
> What does 'release semantics' mean? What is functional difference in both?

'release semantics' is semantics in memory ordering for store operations. 
It ensures store-store ordering.

And some comments below.
> 
> > But on CNXK platform, the functionality of CAS* instructions is completely
> different when it is done to specific addresses. These APIs are meant for use
> for such special cases. These cannot be made ARM generic.
> >
> > Ashwin Sekhar T K
> >
> >> -----Original Message-----
> >> From: Ferruh Yigit <ferruh.yigit@intel.com>
> >> Sent: Tuesday, January 11, 2022 5:42 PM
> >> To: Ashwin Sekhar Thalakalath Kottilveetil <asekhar@marvell.com>;
> >> dev@dpdk.org; Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> >> Ruifeng Wang (Arm Technology China) <Ruifeng.Wang@arm.com>
> >> Cc: Nithin Kumar Dabilpuram <ndabilpuram@marvell.com>; Jerin Jacob
> >> Kollanukkaran <jerinj@marvell.com>; Sunil Kumar Kori
> >> <skori@marvell.com>; Satha Koteswara Rao Kottidi
> >> <skoteshwar@marvell.com>; Pavan Nikhilesh Bhagavatula
> >> <pbhagavatula@marvell.com>; Kiran Kumar Kokkilagadda
> >> <kirankumark@marvell.com>; Satheesh Paul <psatheesh@marvell.com>;
> >> Anoob Joseph <anoobj@marvell.com>; Akhil Goyal <gakhil@marvell.com>
> >> Subject: [EXT] Re: [PATCH] common/cnxk: use cas with release
> >> semantics for batch alloc
> >>
> >> External Email
> >>
> >> ---------------------------------------------------------------------
> >> - On 1/11/2022 12:08 PM, Ferruh Yigit wrote:
> >>> On 11/30/2021 5:45 AM, Ashwin Sekhar T K wrote:
> >>>> Before issuing the batch alloc, we clear the first word of cache
> >>>> lines so that NPA can update the status. Make sure that this line
> >>>> clear is flushed before the batch alloc is issued.
> >>>>
> >>>> Signed-off-by: Ashwin Sekhar T K <asekhar@marvell.com>
> >>>> ---
> >>>>    drivers/common/cnxk/roc_io.h         | 12 ++++++++++++
> >>>>    drivers/common/cnxk/roc_io_generic.h |  9 +++++++++
> >>>>    drivers/common/cnxk/roc_npa.h        |  2 +-
> >>>>    3 files changed, 22 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/drivers/common/cnxk/roc_io.h
> >>>> b/drivers/common/cnxk/roc_io.h index fe5f7f46d0..4f15503c29 100644
> >>>> --- a/drivers/common/cnxk/roc_io.h
> >>>> +++ b/drivers/common/cnxk/roc_io.h
> >>>> @@ -78,6 +78,18 @@ roc_atomic64_cas(uint64_t compare, uint64_t
> >> swap,
> >>>> int64_t *ptr)
> >>>>        return compare;
> >>>>    }
> >>>> +static __plt_always_inline uint64_t roc_atomic64_casl(uint64_t
> >>>> +compare, uint64_t swap, int64_t *ptr) {
> >>>> +    asm volatile(PLT_CPU_FEATURE_PREAMBLE
> >>>> +             "casl %[compare], %[swap], [%[ptr]]\n"
> >>>> +             : [compare] "+r"(compare)
> >>>> +             : [swap] "r"(swap), [ptr] "r"(ptr)
> >>>> +             : "memory");
> >>>> +
> >>>
> >>> out of curiosity, what is the "cas with release semantics"?
> >>> briefly, what is the difference between 'cas' and 'casl'?
> >>
> >> + Honnappa & Ruifeng,

Thanks Ferruh for adding me in this loop.
> >>
> >> Isn't this API Arm wide, instead of being cnxk specific?
> >> Does it make sense to make this API for arm and cnxk use from there?

Yes, CAS operation can be used Arm wide.
Generally, CAS is available via __atomic_compare_exchange/_n() compiler built-ins. This is the way we use
atomic in DPDK. So there is no need to add another generic API.