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

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

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.

> 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).

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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