linux-kernel - Re: [PATCHv12 1/3] rdmacg: Added rdma cgroup controller

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Message-ID: <20160911175235.GB13442@obsidianresearch.com>
Date:   Sun, 11 Sep 2016 11:52:35 -0600
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Leon Romanovsky <leon@...nel.org>,
        Matan Barak <matanb@...lanox.com>,
        Parav Pandit <pandit.parav@...il.com>,
        Tejun Heo <tj@...nel.org>, cgroups@...r.kernel.org,
        linux-doc@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-rdma@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Doug Ledford <dledford@...hat.com>,
        Liran Liss <liranl@...lanox.com>,
        "Hefty, Sean" <sean.hefty@...el.com>,
        Haggai Eran <haggaie@...lanox.com>,
        Jonathan Corbet <corbet@....net>, james.l.morris@...cle.com,
        serge@...lyn.com, Or Gerlitz <ogerlitz@...lanox.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCHv12 1/3] rdmacg: Added rdma cgroup controller

On Sun, Sep 11, 2016 at 07:24:45PM +0200, Christoph Hellwig wrote:
> > > > I've posted some initial work toward a) a while ago, and once we
> > 
> > Did it get merged? Do you have a pointer?
> 
> http://www.spinics.net/lists/linux-rdma/msg31958.html

Right, I remember that. Certainly the right direction

> > However, everything under verbs is not straightforward. The files in
> > userspace are not copies...
> > 
> > user:
> > 
> > struct ibv_query_device {
> >        __u32 command;
> >        __u16 in_words;
> >        __u16 out_words;
> >        __u64 response;
> >        __u64 driver_data[0];
> > };
> > 
> > kernel:
> > 
> > struct ib_uverbs_query_device {
> >         __u64 response;
> >         __u64 driver_data[0];
> > };
> 
> We'll obviously need different strutures for the libibvers API
> and the kernel interface in this case, and we'll need to figure out
> how to properly translate them.  I think a cast, plus compile time
> type checking ala BUILD_BUG_ON is the way to go.

I'm not sure I follow, which would I cast?

BUILD_BUG_ON(sizeof(ibv_query_device) == sizeof(ib_uverbs_cmd_hdr) +
             sizeof(ib_uverbs_query_device))

?

> > I'm thinking the best way forward might be to use a script and
> > transform userspace into:
> > 
> > struct ibv_query_device {
> > 	struct ib_uverbs_cmd_hdr hdr;
> > 	struct ib_uverbs_query_device cmd;
> > };
> 
> That would break the users of the interface.

Sorry, I mean doing this inside rdma-plumbing. Since the change is ABI
identical the modified libibverbs would still be binary compatible
with all providers but not source compatible. Since all kernel
supported providers are in rdma-plumbing we can add the '.cmd.' at the
same time.

The kernel uapi header would stay the same.

> However automatically generating the user ABI from the kernel one
> might still be a good idea in the long run.

My preference would be to try and use the kernel headers directly.

Jason