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

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

> -----Original Message-----
> From: Ruifeng Wang <Ruifeng.Wang@arm.com>
> Sent: Wednesday, January 12, 2022 11:01 AM
> To: Ferruh Yigit <ferruh.yigit@intel.com>; Ashwin Sekhar Thalakalath
> Kottilveetil <asekhar@marvell.com>; dev@dpdk.org; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@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>;
> nd <nd@arm.com>
> Subject: RE: [EXT] Re: [PATCH] common/cnxk: use cas with release semantics
> for batch alloc
> 
> > -----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.

Just to make my comment more clear.
For generic CAS operations, compiler built-ins can be used. No more API needed.
Given the special usage of the instructions in CNXK, the inline assembly here is not intended to be a wrapper of 
generic CAS operation but rather an interface to other hardware function. It doesn't make sense to make it Arm wide.

Thanks.