Re: [PATCH v3] ethdev: fix ethdev data alignment

[Jerin Jacob <jerin.jacob@caviumnetworks.com>]

-----Original Message-----
> Date: Mon, 12 Feb 2018 14:02:17 +0000
> From: Matan Azrad <matan@mellanox.com>
> To: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> CC: "dev@dpdk.org" <dev@dpdk.org>, "ferruh.yigit@intel.com"
>  <ferruh.yigit@intel.com>, "stable@dpdk.org" <stable@dpdk.org>, Thomas
>  Monjalon <thomas@monjalon.net>, Konstantin Ananyev
>  <konstantin.ananyev@intel.com>, Pavan Nikhilesh
>  <pbhagavatula@caviumnetworks.com>
> Subject: RE: [dpdk-dev] [PATCH v3] ethdev: fix ethdev data alignment
> 
> 
> 
> From: Jerin Jacob, Sent: Monday, February 12, 2018 3:51 PM
> > -----Original Message-----
> > > Date: Mon, 12 Feb 2018 13:44:54 +0000
> > > From: Matan Azrad <matan@mellanox.com>
> > > To: Jerin Jacob <jerin.jacob@caviumnetworks.com>, "dev@dpdk.org"
> > >  <dev@dpdk.org>
> > > CC: "ferruh.yigit@intel.com" <ferruh.yigit@intel.com>, "stable@dpdk.org"
> > >  <stable@dpdk.org>, Thomas Monjalon <thomas@monjalon.net>,
> > Konstantin
> > > Ananyev <konstantin.ananyev@intel.com>, Pavan Nikhilesh
> > > <pbhagavatula@caviumnetworks.com>
> > > Subject: RE: [dpdk-dev] [PATCH v3] ethdev: fix ethdev data alignment
> > >
> > > Hi Jerin
> > >
> > > From: Jerin Jacob [mailto:jerin.jacob@caviumnetworks.com]
> > > > The struct rte_eth_dev_data is used in ethdev fastpath routines and
> > > > it not aligned to cache line size. This patch fixes the ethdev data
> > alignment.
> > > >
> > > > The alignment was broken from the "first public release" changeset
> > > > where ethdev data address was aligned only to the first port.
> > > > Remaining ports alignment was defined by the size of the struct
> > > > (rte_eth_dev_data). This scheme is not guaranteed to be cache line
> > > > aligned all the time.
> > > >
> > > > "ethdev: add port ownership" change set introduced a
> > > > rte_eth_dev_shared_data container for port ownership change, This
> > > > resulted in rte_eth_dev->data memory for the first port also as
> > > > cache unaligned.
> > > >
> > > > Added a compiler alignment attribute to make sure rte_eth_dev->data
> > > > always cache aligned so that CPU/compiler
> > > > 1) Avoid sharing the element with another cache line
> > > > 2) Can load/store the elements in struct rte_eth_dev_data as
> > > > naturally aligned.
> > > >
> > > > Some platform like thunderX could see performance regression of 1%
> > > > at
> > > > "ethdev: add port ownership" change set with
> > > > 1 port/1 queue l3fwd application and this patch fixes that regression.
> > > >
> > > > example command:
> > > > sudo ./examples/l3fwd/build/l3fwd -c 0xff00 -- -p 0x1 --config="(0,0,9)"
> > > >
> > > > Fixes: af75078fece3 ("first public release")
> > > > Fixes: 5b7ba31148a8 ("ethdev: add port ownership")
> > >
> > > I don't think you need the add the 5b7ba31148a8 fix line.
> > > Maybe think about it in the next way:
> > > Is your fix can stay as a same fix before the port ownership feature
> > addition?
> > > If yes, You are not fixing it.
> > 
> > I don't think so. 5b7ba31148a8 breaking the first port alignment case(as
> > mentioned in the commit log clearly).
> > First port alignment was correct prior to 5b7ba31148a8 change set
> > 
> > Do you agree?
> 
> No.
> The root cause of the issue this fix is fixing, is that the structure is not aligned, it is not relevant to port ownership.
> The port ownership just exposes it.

My point is simple. First port alignment was correct prior to 5b7ba31148a8 change set.
it is broken from there.

It is minor thing. I let, ethdev maintainer to decide what to added in Fixes on apply.