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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 12/10/16 14:06, Wei Liu wrote:
> 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.

Wouldn't it be just as easy, and more useful, to set a "has been
livepatched" flag, and return errors for all gcov hypercalls if its' set?

I would expect most users to want to build a single hypervisor that can
be used for both gcov testing and live patching (under different
circumstances).

 -George


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