netdev - Re: [PATCH rdma-next 0/4] Introduce dynamic UAR allocation mode

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Date:   Wed, 18 Mar 2020 10:21:00 -0300
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Doug Ledford <dledford@...hat.com>, linux-rdma@...r.kernel.org,
        Michael Guralnik <michaelgur@...lanox.com>,
        netdev@...r.kernel.org, Saeed Mahameed <saeedm@...lanox.com>,
        Yishai Hadas <yishaih@...lanox.com>
Subject: Re: [PATCH rdma-next 0/4] Introduce dynamic UAR allocation mode

On Wed, Mar 18, 2020 at 03:14:50PM +0200, Leon Romanovsky wrote:
> On Wed, Mar 18, 2020 at 09:54:59AM -0300, Jason Gunthorpe wrote:
> > On Wed, Mar 18, 2020 at 02:43:25PM +0200, Leon Romanovsky wrote:
> > > From: Leon Romanovsky <leonro@...lanox.com>
> > >
> > > From Yishai,
> > >
> > > This series exposes API to enable a dynamic allocation and management of a
> > > UAR which now becomes to be a regular uobject.
> > >
> > > Moving to that mode enables allocating a UAR only upon demand and drop the
> > > redundant static allocation of UARs upon context creation.
> > >
> > > In addition, it allows master and secondary processes that own the same command
> > > FD to allocate and manage UARs according to their needs, this can’t be achieved
> > > today.
> > >
> > > As part of this option, QP & CQ creation flows were adapted to support this
> > > dynamic UAR mode once asked by user space.
> > >
> > > Once this mode is asked by mlx5 user space driver on a given context, it will
> > > be mutual exclusive, means both the static and legacy dynamic modes for using
> > > UARs will be blocked.
> > >
> > > The legacy modes are supported for backward compatible reasons, looking
> > > forward we expect this new mode to be the default.
> >
> > We are starting to accumulate a lot of code that is now old-rdma-core
> > only.
> 
> Agree
> 
> >
> > I have been wondering if we should add something like
> >
> > #if CONFIG_INFINIBAND_MIN_RDMA_CORE_VERSION < 21
> > #endif
> 
> From one side it will definitely help to see old code, but from another
> it will create many ifdef inside of the code with a very little chance
> of testing. Also we will continue to have the same problem to decide when
> we can delete this code.

Well, it doesn't have to be an #ifdef, eg just sticking

if (CONFIG_INFINIBAND_MIN_RDMA_CORE_VERSION >= 21)
     return -ENOPROTOOPT;

at the top of obsolete functions would go a long way

> > So we can keep track of what is actually a used code flow and what is
> > now hard to test legacy code.
> >
> > eg this config would also disable the write interface(), turn off
> > compat write interfaces as they are switched to use ioctl, etc, etc.
> 
> What about if we introduce one ifdef, let's say CONFIG_INFINIBAND_LEGACY
> and put everything that will be declared as legacy to that bucket? And
> once every 5 (???) years delete everything from that bucket.

It is much harder to see what is really old vs only a little old

I'm not sure we can ever completely delete any of this, but at least
the distros can make an informed choice to either do more detailed
test of old libraries or disable those code paths.

Jason