[PATCH for-4.22?] x86/EFI: arrange for non-discardable .reloc in xen.efi

[Jan Beulich <jbeulich@xxxxxxxx>]

Loaders respecting IMAGE_SCN_MEM_DISCARDABLE would not load such sections,
yet we need to access it ourselves when switching out of "physical mode".
Leverage behavior new to GNU ld 2.46: Any contribution to .reloc which
doesn't have the discardable flag set (which cannot even be expressed in
ELF) will yield the output section also non-discardable.

Since for intermediate binaries we don't care about section attributes,
link in the new object only on the final linking pass.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
With a new enough linker, $(dot-target).2r.o could be leveraged instead:
$(dot-target).2r.S is going to be empty (or if it's not, the .reloc
section there would already have the wanted property), so instead of
adding the new relocs-empty.S we could also insert the section directive
there. Overall that seemed more convoluted to me, though.

--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -196,7 +196,8 @@ note_file_option ?= $(note_file)
 
 extra-$(XEN_BUILD_PE) += efi.lds
 ifeq ($(XEN_BUILD_PE),y)
-$(TARGET).efi: $(objtree)/prelink.o $(note_file) $(obj)/efi.lds 
$(obj)/efi/relocs-dummy.o $(obj)/efi/mkreloc
+$(TARGET).efi: $(obj)/efi/relocs-dummy.o $(obj)/efi/relocs-empty.o 
$(obj)/efi/mkreloc
+$(TARGET).efi: $(objtree)/prelink.o $(note_file) $(obj)/efi.lds
 ifeq ($(CONFIG_DEBUG_INFO),y)
        $(if $(filter --strip-debug,$(EFI_LDFLAGS)),echo,:) "Will strip debug 
info from $(@F)"
 endif
@@ -227,7 +228,7 @@ endif
        $(MAKE) $(build)=$(@D) .$(@F).2r.o .$(@F).2s.o
        $(call compare-symbol-tables, $(dot-target).1r.o, $(dot-target).2r.o)
        $(call compare-symbol-tables, $(dot-target).1s.o, $(dot-target).2s.o)
-       $(LD) $(call EFI_LDFLAGS,$(VIRT_BASE)) -T $(obj)/efi.lds $< \
+       $(LD) $(call EFI_LDFLAGS,$(VIRT_BASE)) -T $(obj)/efi.lds $< 
$(obj)/efi/relocs-empty.o \
              $(dot-target).2r.o $(dot-target).2s.o $(orphan-handling-y) \
              $(note_file_option) -o $@
        $(NM) -pa --format=sysv $@ \
@@ -247,7 +248,7 @@ $(TARGET).efi: FORCE
 endif
 
 # These should already have been rebuilt when building the prerequisite of 
"prelink.o"
-$(obj)/efi/buildid.o $(obj)/efi/relocs-dummy.o: ;
+$(obj)/efi/buildid.o $(obj)/efi/relocs-dummy.o $(obj)/efi/relocs-empty.o: ;
 
 .PHONY: include
 include: $(objtree)/arch/x86/include/asm/asm-macros.h
--- a/xen/arch/x86/efi/Makefile
+++ b/xen/arch/x86/efi/Makefile
@@ -17,5 +17,5 @@ obj-y := common-stub.o stub.o
 obj-$(XEN_BUILD_EFI) := $(filter-out %.init.o,$(EFIOBJ-y))
 obj-bin-$(XEN_BUILD_EFI) := $(filter %.init.o,$(EFIOBJ-y))
 obj-bin-y += mbi2.init.o
-extra-$(XEN_BUILD_EFI) += buildid.o relocs-dummy.o
+extra-$(XEN_BUILD_EFI) += buildid.o relocs-dummy.o relocs-empty.o
 nocov-$(XEN_BUILD_EFI) += stub.o
--- /dev/null
+++ b/xen/arch/x86/efi/relocs-empty.S
@@ -0,0 +1,8 @@
+/*
+ * Empty .reloc section, simply to indicate to GNU ld that the output .reloc
+ * section in xen.efi should not be marked IMAGE_SCN_MEM_DISCARDABLE.  This
+ * requires GNU ld 2.46 or newer to actually be understood in the intended way.
+ */
+
+       .section .reloc, "a", @progbits
+       .balign 4