Re: [Xen-devel] [PATCH v3] gcov: add new interface and 3.4 and 4.7 format support

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Oct 12, 2016 at 06:42:53AM -0600, Jan Beulich wrote:
> >>> On 11.10.16 at 12:31, <wei.liu2@xxxxxxxxxx> wrote:
> > --- /dev/null
> > +++ b/xen/common/gcov/gcc_4_7.c
> > @@ -0,0 +1,205 @@
> > +/*
> > + *  This code provides functions to handle gcc's profiling data format
> > + *  introduced with gcc 4.7.
> > + *
> > + *  This file is based heavily on gcc_3_4.c file.
> > + *
> > + *  For a better understanding, refer to gcc source:
> > + *  gcc/gcov-io.h
> > + *  libgcc/libgcov.c
> > + *
> > + *  Uses gcc-internal data definitions.
> > + *
> > + *  Imported from Linux and modified for Xen by
> > + *    Wei Liu <wei.liu2@xxxxxxxxxx>
> > + */
> > +
> > +#include <xen/string.h>
> > +
> > +#include "gcov.h"
> > +
> > +#if GCC_VERSION < 40700
> > +#error "Wrong version of GCC used to compile gcov"
> > +#endif
> > +
> > +#if (__GNUC__ > 5) || (__GNUC__ == 5 && __GNUC_MINOR__ >= 1)
> > +#define GCOV_COUNTERS                   10
> > +#elif __GNUC__ == 4 && __GNUC_MINOR__ >= 9
> > +#define GCOV_COUNTERS                   9
> > +#else
> > +#define GCOV_COUNTERS                   8
> > +#endif
> 
> I'm sorry for not having pointed this out on v2 (I had noticed it,
> but then didn't finish analyzing the situation), but I'm afraid this
> together with ...
> 
> > +struct gcov_info {
> > +    unsigned int version;
> > +    struct gcov_info *next;
> > +    unsigned int stamp;
> > +    const char *filename;
> > +    void (*merge[GCOV_COUNTERS])(gcov_type *, unsigned int);
> > +    unsigned int n_functions;
> > +    struct gcov_fn_info **functions;
> > +};
> 
> ... this structure's trailing fields actually getting used by the code
> won't work well when changing compiler versions without cleaning
> the tree.  I think instead you need thin gcc_5.c and gcc_4_9.c
> #define-ing their GCOV_COUNTERS and then #include-ing this
> shared source file. Plus btw, I don't think gcc 5.0.x (the
> development variant of 5.x) would use anything different from
> 5.1.x or 5.2.x; in fact use of __GNUC_MINOR__ should not
> normally be necessary anymore with gcc 5+.
> 

Right. I will do something about this. Thanks for catching this.

> And then - how is all of this supposed to be working in conjucntion
> with live patching, where the patch may have been created by yet
> another compiler version?
> 

There is a version field in gcov_info, so we can compare that and reject
incompatible version.

We need to use hooks in livepatching to call the constructor /
destructor when applying / reverting a live-patch.  We might need to be
cautious about locks or whatever, but I'm sure it can be figured out.

But I have no idea how useful it would be to use gcov and livepatching
together.  For now the easiest thing to do is to

   depends on !LIVEPATCH

in Kconfig.

Wei.

> Jan
> 

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