linux-kernel - Re: [PATCH] ocxl: Add get_metadata IOCTL to share OCXL information to userspace

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Message-Id: <1519271512.2867.14.camel@au1.ibm.com>
Date:   Thu, 22 Feb 2018 14:51:52 +1100
From:   "Alastair D'Silva" <alastair@....ibm.com>
To:     Balbir Singh <bsingharora@...il.com>,
        Frederic Barrat <fbarrat@...ux.vnet.ibm.com>
Cc:     Arnd Bergmann <arnd@...db.de>, frederic.barrat@...ibm.com,
        Greg KH <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...abs.org>,
        Andrew Donnellan <andrew.donnellan@....ibm.com>
Subject: Re: [PATCH] ocxl: Add get_metadata IOCTL to share OCXL information
 to userspace

On Thu, 2018-02-22 at 14:46 +1100, Balbir Singh wrote:
<snip>
> lpc_size could be added. It's currently useless to the library, but
> > doesn't
> > hurt. The one which was giving me troubles on a previous version of
> > this
> > patch was the lpc numa node ID, since that was experimental code
> > and felt
> > out of place considering what's been upstreamed in skiboot and
> > linux so far.
> > 
> 
> Yeah, I think metadata will evolve for a while till it settle's down.
> Since ocxl_ioctl_get_metadata is exposed via uapi, a newer program
> calling an older kernel will never work, since the size of that
> struct
> will always be larger than what the OS supports and our
> copy_to_user()
> will fail. The other option is for the user program to try all
> possible versions till one succeeds, that is bad as well. I think
> there are a few ways around it, if we care about this combination.
> 
> Balbir Singh.
> 

We have a number of reserved members at the end of the struct which can
be re-purposed for future information (with a corresponding bump of the
version number).

-- 
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australia
mob: 0423 762 819