Re: [PATCH v1 2/2] Strip build path directories in tools, xen and xen/arch/x86

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.03.2025 16:58, Marek Marczykowski-Górecki wrote:
> On Thu, Mar 20, 2025 at 04:48:02PM +0100, Jan Beulich wrote:
>> On 20.03.2025 16:32, Marek Marczykowski-Górecki wrote:
>>> On Thu, Mar 20, 2025 at 04:21:18PM +0100, Jan Beulich wrote:
>>>> On 20.03.2025 16:17, Marek Marczykowski-Górecki wrote:
>>>>> On Thu, Mar 20, 2025 at 02:35:59PM +0000, Anthony PERARD wrote:
>>>>>> On Thu, Mar 20, 2025 at 02:59:04PM +0100, Marek Marczykowski-Górecki 
>>>>>> wrote:
>>>>>>> On Thu, Mar 20, 2025 at 02:49:27PM +0100, Jan Beulich wrote:
>>>>>>>> On 20.03.2025 13:51, Marek Marczykowski-Górecki wrote:
>>>>>>>>> On Thu, Mar 20, 2025 at 10:18:28AM +0000, Anthony PERARD wrote:
>>>>>>>>>> On Wed, Mar 19, 2025 at 02:40:33PM +0100, Marek Marczykowski-Górecki 
>>>>>>>>>> wrote:
>>>>>>>>>>> There are clearly some build path embedding left. And
>>>>>>>>>>> -ffile-prefix-map=/-fdebug-prefix-map= doesn't work correctly with
>>>>>>>>>>> XEN_ROOT having xen/.. at the end.
>>>>>>>>>>> BTW, would it be acceptable to have this?
>>>>>>>>>>>
>>>>>>>>>>>     $(call cc-option-add,CFLAGS,CC,-fdebug-prefix-map=$(realpath 
>>>>>>>>>>> $(XEN_ROOT))=.)
>>>>>>>>>>
>>>>>>>>>> Hi,
>>>>>>>>>>
>>>>>>>>>> Could you avoid using $(XEN_ROOT) in hypervisor build system? (It's 
>>>>>>>>>> fine
>>>>>>>>>> in "tools/"). In "xen/", there's a few variables you can use if they 
>>>>>>>>>> are
>>>>>>>>>> needed: $(abs_objtree) $(abs_srctree) for absolutes path, and 
>>>>>>>>>> $(srctree)
>>>>>>>>>> $(objtree) for relative path. That also should avoid the need to use
>>>>>>>>>> $(realpath ).
>>>>>>>>>
>>>>>>>>> XEN_ROOT is literally "$(abs_srctree)/..". And I need to resolve it to
>>>>>>>>> not have /.. for prefix-map to work correctly. Would it be better to 
>>>>>>>>> use
>>>>>>>>> literal $(realpath $(abs_srctree)/..)? Or use just $(abs_srctree) and
>>>>>>>>> have paths in debug symbols relative to hypervisor source dir, instead
>>>>>>>>> of xen repo root? I'm not sure if that wouldn't confuse some tools...
>>>>>>>>
>>>>>>>> abs_srctree being computed using realpath, can't we replace
>>>>>>>>
>>>>>>>> export XEN_ROOT := $(abs_srctree)/..
>>>>>>>>
>>>>>>>> by something as simpl{e,istic} as
>>>>>>>>
>>>>>>>> export XEN_ROOT := $(patsubst %/xen,%,$(abs_srctree))
>>>>>>>>
>>>>>>>> ?
>>>>>>>
>>>>>>> That works too. It's slightly less robust, but I don't expect "xen"
>>>>>>> directory to be renamed, so shouldn't be an issue. I'll leave also a
>>>>>>> comment there why not /.. .
>>>>>>
>>>>>> I don't think $(XEN_ROOT) is present in the binaries produce by the
>>>>>> hypervisor's build system. There's only a few use if that variable: to
>>>>>> load some makefile, to execute makefile that build xsm policy and to
>>>>>> generate cpuid-autogen.h. Otherwise I don't think the compile have this
>>>>>> path in the command line. What is going to be in the binary is
>>>>>> $(abs_srctree), which you can replace by "./xen" if you want; which mean
>>>>>> it doesn't matter if the directory is renamed or not. You might want to
>>>>>> also take care of $(abs_objtree) which seems to also be in `xen-syms`
>>>>>> when doing out-of-tree build.
>>>>>
>>>>> So, you suggest to do -fdebug-prefix-map=$(abs_srctree)=./xen ? That
>>>>> appears to work for in-tree builds too.
>>>>
>>>> And why ./xen (question to Anthony)? Just . is quite fine, isn't it?
>>>
>>> It makes paths in debug symbols relative to xen/ subdir, not the
>>> repository root. I'm not sure if that is a problem, but it may be for
>>> some tools.
>>
>> Yet especially in the symbol table (and hence in strack traces) that's
>> unnecessary extra space it takes up.
>>
>>>>> But now I actually tested how it looks with out-of-tree builds, and
>>>>> indeed $(abs_objtree) is embedded there too. Adding
>>>>> -fdebug-prefix-map=$(abs_objtree)=./xen appears to help for this. But,
>>>>> -fdebug-prefix-map doesn't help with abs_srctree in out-of-tree builds
>>>>> for some reason. -ffile-prefix-map does. And so does -fdebug-prefix-map
>>>>> + -fmacro-prefix-map. Is there any preference which one to use? It
>>>>> appears as -fmacro-prefix-map and -ffile-prefix-map have the same
>>>>> availability in both GCC (8) and Clang (10).
>>>>
>>>> Then the simpler -ffile-prefix-map is better, imo. Question then is
>>>> whether any of the options is actually needed at all for in-tree builds.
>>>
>>> Yes, without any of those options, both xen-syms and xen.efi contain
>>> full source path.
>>
>> Even in builds without debug info? 
> 
> For in-tree build without debug info, it appears no. But with debug
> info, something is needed even for in-tree build.
> And BTW, IIUC out-of-tree builds will become relevant even for in-tree
> build at some point, due to pvshim.

That hasn't happened yet because it's not quite straightforward to arrange
for.

>> Imo a goal ought to be to specify the
>> weakest possible of these options for any particular build mode. I.e.
>> possibly -ffile-prefix-map= for out of tree builds, else
>> -fdebug-prefix-map= when DEBUG_INFO=y, else nothing (if possible).
> 
> Is it? I don't really see why making the selection overly complex if the
> option is supported (and cc-option-add covers that case).

Yes, cc-option-add might cover the case where nothing is needed. But the
two options mentioned have appeared in gcc at different versions. People
using e.g. gcc7 may still benefit from -fdebug-prefix-map=.

Jan