Re: [dpdk-dev] [PATCH v2 1/3] eal/arm64: relax the io barrier for aarch64

[Gavin Hu <Gavin.Hu@arm.com>]

Hi Jerin,

> -----Original Message-----
> From: Jerin Jacob <jerinjacobk@gmail.com>
> Sent: Thursday, January 2, 2020 5:52 PM
> To: Gavin Hu <Gavin.Hu@arm.com>
> Cc: dpdk-dev <dev@dpdk.org>; nd <nd@arm.com>; David Marchand
> <david.marchand@redhat.com>; thomas@monjalon.net;
> rasland@mellanox.com; maxime.coquelin@redhat.com; tiwei.bie@intel.com;
> hemant.agrawal@nxp.com; jerinj@marvell.com; Pavan Nikhilesh
> <pbhagavatula@marvell.com>; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; Ruifeng Wang
> <Ruifeng.Wang@arm.com>; Phil Yang <Phil.Yang@arm.com>; Joyce Kong
> <Joyce.Kong@arm.com>; Steve Capper <Steve.Capper@arm.com>
> Subject: Re: [dpdk-dev] [PATCH v2 1/3] eal/arm64: relax the io barrier for
> aarch64
> 
> On Mon, Dec 23, 2019 at 3:46 PM Gavin Hu <Gavin.Hu@arm.com> wrote:
> >
> > Hi Jerin,
> >
> > > -----Original Message-----
> > > From: Jerin Jacob <jerinjacobk@gmail.com>
> > > Sent: Monday, December 23, 2019 5:20 PM
> > > To: Gavin Hu <Gavin.Hu@arm.com>
> > > Cc: dpdk-dev <dev@dpdk.org>; nd <nd@arm.com>; David Marchand
> > > <david.marchand@redhat.com>; thomas@monjalon.net;
> > > rasland@mellanox.com; maxime.coquelin@redhat.com;
> > > tiwei.bie@intel.com; hemant.agrawal@nxp.com; jerinj@marvell.com;
> > > Pavan Nikhilesh <pbhagavatula@marvell.com>; Honnappa Nagarahalli
> > > <Honnappa.Nagarahalli@arm.com>; Ruifeng Wang
> > > <Ruifeng.Wang@arm.com>; Phil Yang <Phil.Yang@arm.com>; Joyce Kong
> > > <Joyce.Kong@arm.com>; Steve Capper <Steve.Capper@arm.com>
> > > Subject: Re: [dpdk-dev] [PATCH v2 1/3] eal/arm64: relax the io barrier for
> > > aarch64
> > >
> > > On Mon, Dec 23, 2019 at 2:44 PM Gavin Hu <Gavin.Hu@arm.com> wrote:
> > > >
> > > > Hi Jerin,
> > >
> > > Hi Gavin,
> > >
> > > >
> > > > I think we are on the same page with regard to the problem, and the
> > > situations, thanks for illuminating the historical background of the two
> > > barriers.
> > > > About the solution, I added inline comments.
> > > > > It will be optimization only when if we are changing in the fast path.
> > > > > In the slow path, it does not matter.
> > > > > I think, the First step should be to use rte_cio_* wherever it is
> > > > > coherent memory used in _fast path_. I think, Almost every driver
> > > > > fixed that.
> > > > >
> > > > > I am not against this patch(changing the slow path to use rte_cio*
> > > > > from rte_io* and virtio changes associated with that).
> > > > > If you are taking that patch, pay attention to all the drivers in the
> > > > > tree which is using rte_io* for mixed access in slowpath.
> > > > I see 30+ drivers has calling rte_io* directly or indirectly through
> > > rte_write/read*.
> > > > It is hard for me to figure out all the mixed accesses in these drivers, and
> > > as you said, it makes no sense to change the _slow path_.
> > > >
> > > > How about we keep the old rte_io as is, and introduce 'fast path' version
> > > of rte_io for new code use?
> > > > Then in future, we may merge the two?
> > > > Another reason about this proposal is maybe there is rte_io calling in
> the
> > > fast path, but they are not mixed accesses and rte_cio is not suitable.
> > >
> > > Could you share more details about the case where fastpath + rte_io
> > > needed + rte_cio is not suitable?
> >
> > Here is an example for i40e, in the fast path, but only a pure io memory
> access.
> >
> https://code.dpdk.org/dpdk/v19.11/source/drivers/net/i40e/i40e_rxtx.c#L12
> 08
> 
> Yes. That's a performance issue.
> 
> It could be changed to following for the fix that works on x86, arm64
> with existing infra.
> 
> From:
> I40E_PCI_REG_WRITE()
> 
> to:
> 
> rte_cio_wmb()
> I40E_PCI_REG_WRITE_RELAXED()
Yes, this is correct, I will submit a new patch for this.
This is an example out of all the cases that I must fix before relaxing the rte_io barriers.
My plan is as follows, any comments are welcome!
1. replace rte_*mb and rte_io_*mb with rte_cio_*mb where applicable in the fastpath, this is an optimization, as the barriers are relaxed.
2. replace all the rte_io_*mb with rte_cio_*mb where applicable in the slowpath and control path
3. until *all* the occurrences in the step 1 and 2 are done, then this path can be re-activated.

Please advise if the above approach works from your viewpoint.
Maybe I will stop at step 1, step 2 and 3 are not necessary as they are not in the fastpath? 

> 
> >
> > I wanted two variants of rte_io, because also x86 requires two as indicated
> here, one for no-WC and another for WC.
> > http://inbox.dpdk.org/dev/20191204151916.12607-1-
> xiaoyun.li@intel.com/T/#ea8bb1b4a378ab09baedbf95b4542bcb92f4a396f
> > >
> > > >
> > > > Any thoughts?
> > > >
> > > > >
> > > > > > But as the case in i40e, we must pay attention to where rte_cio was
> > > > > missing but rescued by old rte_io(but not by new rte_io).
> > > > > >
> > > > > >