linux-kernel - Re: [RFC PATCH 1/9] cxl/mem: Implement Get Event Records command

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Date:   Wed, 21 Sep 2022 21:16:01 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
CC:     Dave Jiang <dave.jiang@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        <linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>
Subject: Re: [RFC PATCH 1/9] cxl/mem: Implement Get Event Records command

On Wed, Sep 21, 2022 at 05:36:42PM +0100, Jonathan Cameron wrote:
> On Tue, 20 Sep 2022 15:10:26 -0700
> Ira Weiny <ira.weiny@...el.com> wrote:
> 
> > On Tue, Sep 20, 2022 at 01:23:29PM -0700, Jiang, Dave wrote:

[snip]

> > >
> > > And what gets printed
> > > to the trace buffer can be decoded data constructed from those fields by the
> > > kernel code. So with that you can have access both.
> > >   
> > 
> > Fast assigning the entire buffer + decoded versions will roughly double the
> > trace event size.
> > 
> > Thinking through this a bit more there is a sticking point.
> > 
> > The difficulty will be ensuring that any new field names are documented such
> > that when user space starts to look at them they can determine if that data
> > appears as a new field or as part of a reserved field.
> > 
> > For example if user space needs to access data in the reserved data now it can
> > simply decode it.  However, when that data becomes a field it no longer is part
> > of the reserved data.  So what user space would need to do is look for the
> > field first (ie know the field name) and then if it does not appear extract it
> > from the reserved data.
> > 
> > I'm now wondering if I've wasted my time decoding anything since the kernel
> > does not need to know anything about these fields.  Because the above scenario
> > means that user space may get ugly over time.
> > 
> > That said I don't think it will present any incompatibilities.  So perhaps we
> > are ok?
> 
> I favor decoding current record in kernel and packing it appropriately.
> If that means we don't provide some new data from a future version then such
> is life - the kernel needs upgrading.  That information is unlikely to be
> crucial - it's probably just more detail.

Dave, Dan, and I discussed this further today.  Dan expressed the same opinion.
So I'm going to remove all the reserved fields from the next version.

Thanks,
Ira