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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
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