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 05:09:12PM +0100, Jan Beulich wrote:
> 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.

Sure, but if it will happen at some point, even users doing in-tree
build would benefit from options that normally would be relevant only
for out-of-tree builds. So, it's IMO valuable to attempt make
out-of-tree builds reproducible too.

> >> 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=.

That sounds like an argument to use -fdebug-prefix-map= +
-fmacro-prefix-map (with separate cc-option-add), instead of just
-ffile-prefix-map. I'm fine with that.


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

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