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 14:24, George Dunlap wrote:
> 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).

I mean software provider, not user, of course.  That's what I would want
for CentOS, and I'm sure that's what the XenServer (and probably Oracle)
guys want as well.

 -George


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