Re: [XEN PATCH v4 14/18] xen,symbols: rework file symbols selection

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.04.2020 16:42, Anthony PERARD wrote:
> On Fri, Apr 17, 2020 at 03:39:48PM +0200, Jan Beulich wrote:
>> On 17.04.2020 15:19, Anthony PERARD wrote:
>>> Or do you mean keeping exception to the rule? And hope that when someone
>>> changes the rule, it doesn't forget to check if the exception needs
>>> changing as well?
>>
>> ... "exception" like you put it (requiring special care to keep
>> multiple instances in sync) is not the only way this can be done
>> (and indeed I'd not want something like this). Since you have
>> (in patch 15) e.g.
>>
>> guest_walk_%.o: guest_walk.c FORCE
>>      $(call if_changed_rule,cc_o_c)
>>
>> anyway, the desire to skip the objcopy step could be communicated
>> to the command from here, without needing to clone the command.
>> One way might be a special (phony) dependency, another might be to
>> set some variable along the lines of
>>
>> guest_walk_%.o: SPECIAL := y
> 
> I guess something like that could be done. But if possible, I'd like to
> avoid that.
> 
>>> Also, I'm going to have to use this patch later anyway as sometime CC
>>> use a full path to the source as file symbol. So this is going to be
>>> important when we will run for example
>>> `clang -o arch/x86/mm/guest_walk_2.o arch/x86/mm/guest_walk.c`.
>>> (There isn't a patch for that yet.)
>>
>> That's interesting - what will be the goal of that future adjustment?
> 
> It's a step toward my goal of been able to have out-of-tree build for
> xen, as stated in my cover letter. In order to do that, I try to adapt
> Kbuild to build Xen.
> 
> Kbuild is building the linux kernel without changing directory, so I'd
> like to do the same, as it probably makes it easier to do out-of-tree
> build.
> 
> Another tool I'd like to use from Kbuild is ./fixdep, it's a small
> program that run after running CC and fix the dependency file that CC
> generates. The main thing it does is to add a dependency on
> Kconfig options that a source file uses instead of having a dependency
> on whether any unrelated Kconfig has change at all. But ./fixdep from
> Linux only works if we build without changing directory. ([1] for more
> on fixdep)
> 
> I guess one advantage of never changing directory is that we can always
> use relative path in global *FLAGS. There isn't a need to use absolute
> path, which is an issue when the source tree is moved to a different
> location. That can easily happen when for example you try to build in a
> container (mapping the source tree inside it) then try to rebuild from
> outside. (After using automation/scripts/containerize for example.)
> And we don't need tricks like the .*.d2 files (which isn't needed in the
> hypervisor anyway, so far at least).

Ah, I see. Out-of-tree builds don't necessarily imply source trees
that can also be moved, so you want to actually go a step further.

Jan