Re: [Xen-devel] [PATCH v2 0/6] Remove dependency on __LINE__

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On 03/10/2017 08:50 AM, Jan Beulich wrote:
> > > > On 10.03.17 at 09:29, <ross.lagerwall@xxxxxxxxxx> wrote:
> > On 03/09/2017 10:34 AM, Jan Beulich wrote:
> > > > > > On 08.03.17 at 18:46, <ross.lagerwall@xxxxxxxxxx> wrote:
> > > > Sorry for the long delay since the first version of this series
> > > > (previously called "Make building xSplice patches easier").  Here is a
> > > > set of changes that remove the use of __LINE__ when building with NDEBUG
> > > > and LivePatch enabled.  Tested to boot on x86.  Compile-tested on arm.
> > > What I'm missing here is an evaluation of possible alternatives, as
> > > converting from __LINE__ to code addresses implies an extra step
> > > when wanting to look up the place where a problem occurred,
> > > which I generally consider undesirable. For example, I had briefly
> > > discussed with Andrew the addition of #line directives, but iirc he
> > > had indicated some issue with that. Furthermore there is the
> > > option (at least for some of the more simple patches) of squeezing
> > > more than one statement on a single line, perhaps just for patch
> > > creation. And I guess one can think of more.
> > Both of the options you suggested require extra work when building a
> > live patch. This series is intended to remove that extra work as the
> > process is already more difficult than I'd like.
> >
> > I don't think using addr2line is much harder than looking up the line
> > number directly, especially as it is something that needs to be done for
> > many other crashes.
> I agree with the second half, but I don't think I do for the first part:
> I don't know how you would manage this, but for me this is several
> steps:
> - Locate and download the correct package (it's not reasonable to
>   keep them all, there simply are too many)
> - extract xen-syms or xen.efi (and of course I will need to know
>   which one was actually used)
> - run addr2line on it
> Quite a bit more than just going to the source file and finding the
> line right away - in most cases line numbers, as opposed to code
> addresses, don't change much despite a delta of a few dozen
> backports or other patches, so normally one doesn't even need
> to look at the _precise_ tree.
>
> > On your suggestion, use of __LINE__ is disabled only
> > when CONFIG_LIVEPATCH=y and NDEBUG which means that in a typical
> > development scenario, you'd still get line numbers.
> That's understood, but I'm concerned about extra overhead when
> looking at customer reports.
>
> > They would be
> > removed only for "release" builds in which it is likely that the source
> > code & debuginfo is archived somewhere such that looking up a line
> > number requires several steps anyway. I could suggest making it a
> > separate config option but IIRC you prefer to limit the number of config
> > options.
> I prefer to limit ones with non-EXPERT-visible prompts, yes. But
> I also don't see how a separate config option would improve the
> situation.
My suggestion was to have a separate config option so that the person 
configuring Xen could choose in the trade off between having line 
numbers in debug messages or easier live patching. Does that seem 
reasonable?
--
Ross Lagerwall

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