linux-kernel - Re: [PATCH] cxl/pci: Get rid of pointer arithmetic reading CDAT table

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Message-ID: <657bd741d2961_269bd294d@dwillia2-mobl3.amr.corp.intel.com.notmuch>
Date: Thu, 14 Dec 2023 20:34:09 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Robert Richter <rrichter@....com>, Dan Williams <dan.j.williams@...el.com>
CC: Ira Weiny <ira.weiny@...el.com>, Davidlohr Bueso <dave@...olabs.net>,
	Jonathan Cameron <jonathan.cameron@...wei.com>, Dave Jiang
	<dave.jiang@...el.com>, Alison Schofield <alison.schofield@...el.com>, Vishal
 Verma <vishal.l.verma@...el.com>, <linux-cxl@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Fan Ni <nifan.cxl@...il.com>, Lukas Wunner
	<lukas@...ner.de>
Subject: Re: [PATCH] cxl/pci: Get rid of pointer arithmetic reading CDAT table

Robert Richter wrote:
> On 17.11.23 21:09:18, Robert Richter wrote:
> > I will send an on-top patch for 6.8 that reworks that code area to
> > remove the pointer arithmetic.
> 
> Here it is:
> 
> From 13787f72c20b8c54754ae86015d982307eae0397 Mon Sep 17 00:00:00 2001
> From: Robert Richter <rrichter@....com>
> Subject: [PATCH] cxl/pci: Get rid of pointer arithmetic reading CDAT table
> 
> Reading the CDAT table using DOE requires a Table Access Response
> Header in addition to the CDAT entry. In current implementation this
> has caused offsets with sizeof(__le32) to the actual buffers. This led
> to hardly readable code and even bugs (see fix of devm_kfree() in
> read_cdat_data()).
> 
> Rework code to avoid calculations with sizeof(__le32). Introduce
> struct cdat_doe for this which contains the Table Access Response
> Header and a variable payload size for various data structures
> afterwards to access the CDAT table and its CDAT Data Structures
> without recalculating buffer offsets.

I like reworking the code to introduce an explicit type for the response
buffer, but as Ira points out, lets call it a "response" not a
"cdat_doe".

The feedback on the flex array is accurate, but I see no reason to have
3 flex arrays vs:

struct cdat_response {
       __le32 doe_header;
       union {
               struct cdat_header header;
               struct cdat_entry_header entry;
               u8 table[];
       };
} __packed;

As far as I can see nothing outside of drivers/cxl/core/pci.c needs
'struct cdat_response', so it can stay local to this C file.

While you are working on that I will do another lead-in cleanup to kill
the goto in cxl_cdat_read_table() and let you come back and kill off the
open-coded "+ sizeof(__le32)" that I will leave behind.