Re: [Xen-devel] [PATCH] gcov: Support gcc 4.7

[Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>]

On Tue, 2013-06-18 at 15:53 +0100, George Dunlap wrote:
> On 06/18/2013 03:32 PM, Frediano Ziglio wrote:
> > On Mon, 2013-06-17 at 11:49 +0100, Ian Campbell wrote:
> >> On Mon, 2013-06-17 at 11:46 +0100, Frediano Ziglio wrote:
> >>> Perhaps a simple easy solution would be to allow privileged domains to
> >>> read/write arbitrary memory from Xen (write to reset counters).
> >>
> >> "Arbitrary memory" would be a very hard sell.
> >>
> >> If all these counters could be constrained to pages which contains these
> >> counters and nothing else then that might be a possibility to consider.
> >>
> >> Ian.
> >>
> >
> > It depends on how this would be implemented. A future option of gcc
> > could help at least having everything in contiguous memory regions.
> >
> > The fact that these information are generated by the compiler does not
> > help that much. Every version could change the format "slightly" but
> > enough to make Xen core when such information are requested!
> >
> > Actually they changed format for these version of gcc:
> > - 3.3
> > - 3.4
> > - 4.7
> >
> > And honestly I still don't understand clearly why they changed for gcc
> > 4.7! Probably not for a C feature but to support C++ templates and
> > comdat. A "smart" preprocessor sit in the middle of gcc and gas that
> > process assembly generated, check for structures and put everything in
> > different sections could really help... but is not that easy project!
> > Another similar way would be to parse the xen-syms output file and
> > generate needed blob/code. Quite interesting challenge (I did more
> > terrible things then this) but I don't thing that Citrix would agree.
> > Possibly a safe thing to do would be in the code to check that compiler
> > is really gcc and that we can support the version that is compiling,
> > just to make sure. But is also true that this feature is a debug one
> > disabled on production so I don't know how worth would be.
> >
> > Not speaking of clang support, currently quite hard (the structures are
> > less documented, the code hooks destructors path which is not safe to
> > call from Xen code which is not expected to terminate).
> 
> Doesn't Linux have gcov support via lcov?  Do they have to deal with 
> this stuff as well, or is there a generic way to deal with it?
> 
>   -George

They only support gcc 3.4 format (the one we currently support without
my new patch which add support for 4.7 format)

Frediano



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