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

From: Jason Gunthorpe <[email-masked]>
Date: Wed, 18 Mar 2020 11:00:01 -0300

On Wed, Mar 18, 2020 at 03:56:31PM +0200, Leon Romanovsky wrote:
> On Wed, Mar 18, 2020 at 10:21:00AM -0300, Jason Gunthorpe wrote:
> > 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 <[email-masked]>
> > > > >
> > > > > 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
> 
> First, how will you set this min_version? hordcoded in the kernel
> code?

Yes, when a rdma-core release obsoletes the code path then it can
become annotated.

> Second, it will work for simple flows, but can be extremely complex
> if your code looks like:
> if (old_version)
>  do something
> if (new version)
>  do something else

Well, we'd avoid making such complications, it would be something like

if (flag & foo) {
   if (CONFIG_INFINIBAND_MIN_RDMA_CORE_VERSION >= 21)
      return -ENOPROTOOPT;
  [keep going as before]
}

At least we now know this conditional path isn't used / isn't covered
by testing

Doug? What does a distro think?

Jason