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

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Message-ID: <CAKTCnzkjbNVxBVTKmffbxsyqMb83y2tXN1Xv4Zm3VT+sxePdpg@mail.gmail.com>
Date:   Thu, 22 Feb 2018 16:32:51 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     "Alastair D'Silva" <alastair@....ibm.com>
Cc:     Frederic Barrat <fbarrat@...ux.vnet.ibm.com>,
        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, Feb 22, 2018 at 2:51 PM, Alastair D'Silva <alastair@....ibm.com> wrote:
> 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).

Good point, agreed

Balbir Singh.