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

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Wed, Mar 19, 2025 at 01:43:59PM +0100, Jan Beulich wrote:
> On 19.03.2025 12:58, Marek Marczykowski-Górecki wrote:
> > On Wed, Mar 19, 2025 at 10:43:12AM +0100, Jan Beulich wrote:
> >> On 19.03.2025 10:15, Jan Beulich wrote:
> >>> On 18.03.2025 18:01, Marek Marczykowski-Górecki wrote:
> >>>> --- a/xen/Makefile
> >>>> +++ b/xen/Makefile
> >>>> @@ -411,6 +411,8 @@ ifneq ($(CONFIG_CC_IS_CLANG),y)
> >>>>  CFLAGS += -Wa,--strip-local-absolute
> >>>>  endif
> >>>>  
> >>>> +$(call cc-option-add CFLAGS,CC,-ffile-prefix-map=$(XEN_ROOT)=.)
> >>>
> >>> This is lacking a comma:
> >>>
> >>> $(call cc-option-add,CFLAGS,CC,-ffile-prefix-map=$(XEN_ROOT)=.)
> >>
> >> And then, having tried the correct form (seeing the option then is passed
> >> to the compiler), I can't spot any difference in the resulting
> >> xen-syms.map. There were a few absolute paths there before (for
> >> arch/x86/x86_64/kexec_reloc.S and arch/x86/acpi/wakeup_prot.S), and the
> >> exact same ones are present afterwards.
> > 
> > I'm not sure about xen-syms.map, it's about build path included in
> > xen-syms. It appears at least once in .debug_str and once in
> > .debug_line_str.
> 
> In which case -fdebug-prefix-map= may suffice, which is available on
> more compiler versions? And then only if DEBUG_INFO=y?

Oh, and xen.efi is full of build path. Binary on plain staging has 790
occurrences. But there, -fdebug-prefix-map= also helps.

But also I don't think -fdebug-prefix-map= will be enough for tools, it
looks like at least libxl has build path embedded in .rodata too.

> > But also, I see the patch lost a chunk during rebase (from before
> > 4.17...), that adjusts XEN_ROOT to use $(realpath ...). That's the part
> > even mentioned in the commit message...
> > 
> > I'll send v2 shortly.
> 
> Provided there's actually a need. I was in fact wondering whether this
> was known to have significant effect prior to Anthony's work to make
> out-of-tree builds possible (plus less related adjustments), but may
> have lost most of its functionality since then (yet was still carried
> forward for all the time).

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))=.)

It may be less efficient (if make doesn't cache result), but helps
especially in tools, where XEN_ROOT is set in _a lot_ of places.

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

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