Re: [PATCH] RFC: xen/x86: Enable --gc-sections

[Jason Andryuk <jason.andryuk@xxxxxxx>]

On 2025-12-08 13:45, Grygorii Strashko wrote:
> On 06.12.25 00:28, Jason Andryuk wrote:
> > When linking to create xen-syms, add --gc-sections to garbage collect
> > unused stuff.  Relies on CONFIG_CC_SPLIT_SECTIONS
> >
> > We need to add KEEP() to the linker script in assorted places to retain
> > appropriate data - especially the arrays created therein.
> >
> > Something is off though.  In a test where memory_add() is unreachable,
> > it is still included.  I'm not sure, but I am wondering if it's the
> > alternatives somehow keeping a reference to it.
> >
> > Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> > ---
> > With --print-gc-sections on defconfig:
> > ld: removing unused section '.text.__bitmap_full' in file 'prelink.o'
> > ld: removing unused section '.text.__bitmap_xor' in file 'prelink.o'
> > ld: removing unused section '.text.__bitmap_set' in file 'prelink.o'
> > ld: removing unused section '.text.__bitmap_clear' in file 'prelink.o'
> > ld: removing unused section '.text.bitmap_find_free_region' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.bitmap_release_region' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.domain_has_ioreq_server' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.compat_kexec_op' in file 'prelink.o'
> > ld: removing unused section '.text.in_atomic' in file 'prelink.o'
> > ld: removing unused section '.text.radix_tree_next_hole' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.radix_tree_prev_hole' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.radix_tree_gang_lookup_slot' in 
> > file 'prelink.o'
> > ld: removing unused section '.text._nrspin_trylock' in file 'prelink.o'
> > ld: removing unused section '.text.xen_compile_host' in file 'prelink.o'
> > ld: removing unused section '.text.vscnprintf' in file 'prelink.o'
> > ld: removing unused section '.text.wake_up_one' in file 'prelink.o'
> > ld: removing unused section '.text.xmem_pool_get_used_size' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.xmem_pool_get_total_size' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.xmem_pool_maxalloc' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.xlat_start_info' in file 'prelink.o'
> > ld: removing unused section '.text.elf_sym_by_name' in file 'prelink.o'
> > ld: removing unused section '.text.elf_sym_by_index' in file 'prelink.o'
> > ld: removing unused section '.text.elf_get_ptr' in file 'prelink.o'
> > ld: removing unused section '.text.elf_lookup_addr' in file 'prelink.o'
> > ld: removing unused section '.text.serial_vuart_info' in file 'prelink.o'
> > ld: removing unused section '.text.pci_find_next_cap' in file 'prelink.o'
> > ld: removing unused section '.text.free_hvm_irq_dpci' in file 'prelink.o'
> > ld: removing unused section '.text.mce_barrier_init' in file 'prelink.o'
> > ld: removing unused section '.text.mce_barrier_dec' in file 'prelink.o'
> > ld: removing unused section '.text.mce_barrier' in file 'prelink.o'
> > ld: removing unused section '.text.apei_read_mce' in file 'prelink.o'
> > ld: removing unused section '.text.apei_check_mce' in file 'prelink.o'
> > ld: removing unused section '.text.apei_clear_mce' in file 'prelink.o'
> > ld: removing unused section '.text.efi_halt_system' in file 'prelink.o'
> > ld: removing unused section '.text.get_vvmcs_virtual_safe' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.get_vvmcs_real_safe' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.set_vvmcs_real' in file 'prelink.o'
> > ld: removing unused section '.text.set_vvmcs_virtual_safe' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.set_vvmcs_real_safe' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.domain_set_alloc_bitsize' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.watchdog_enabled' in file 'prelink.o'
> > ld: removing unused section '.text.unset_nmi_callback' in file 
> > 'prelink.o'
> > ld: removing unused section '.text.sha2_256_init' in file 'prelink.o'
> > ld: removing unused section '.text.xxh64_copy_state' in file 'prelink.o'
> > ld: removing unused section '.text.xxh64' in file 'prelink.o'
> > ld: removing unused section '.discard' in file 'prelink.o'
> > ld: removing unused section '.rodata.xen_compile_host.str1.1' in file 
> > 'prelink.o'
> > ld: removing unused section '.rodata.elf_lookup_addr.str1.1' in file 
> > 'prelink.o'
> > ld: removing unused section '.rodata.apei_read_mce.str1.8' in file 
> > 'prelink.o'
> > ld: removing unused section '.rodata.efi_halt_system.str1.8' in file 
> > 'prelink.o'
> > ld: removing unused section '.rodata.play_dead.str1.1' in file 
> > 'prelink.o'
> > ld: removing unused section '.data.rel.ro.local.fetch_type_names' in 
> > file 'prelink.o'
> > ---
> >   xen/arch/x86/Makefile     |  3 +++
> >   xen/arch/x86/xen.lds.S    | 38 +++++++++++++++++++-------------------
> >   xen/include/xen/xen.lds.h | 18 +++++++++---------
> >   3 files changed, 31 insertions(+), 28 deletions(-)
> >
> > diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
> > index 300cc67407..934c79411a 100644
> > --- a/xen/arch/x86/Makefile
> > +++ b/xen/arch/x86/Makefile
> > @@ -137,18 +137,21 @@ $(TARGET)-syms: $(objtree)/prelink.o $(obj)/xen.lds
> >       $(objtree)/tools/symbols $(all_symbols) --empty > $(dot-target).0.S
> >       $(MAKE) $(build)=$(@D) $(dot-target).0.o
> >       $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
> > +          --gc-sections \
> >             $(dot-target).0.o -o $(dot-target).0
> >       $(NM) -pa --format=sysv $(dot-target).0 \
> >           | $(objtree)/tools/symbols $(all_symbols) --sysv --sort \
> >           > $(dot-target).1.S
> >       $(MAKE) $(build)=$(@D) $(dot-target).1.o
> >       $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
> > +          --gc-sections \
> >           $(dot-target).1.o -o $(dot-target).1
> >       $(NM) -pa --format=sysv $(dot-target).1 \
> >           | $(objtree)/tools/symbols $(all_symbols) --sysv --sort 
> > $(syms-warn-dup-y) \
> >           > $(dot-target).2.S
> >       $(MAKE) $(build)=$(@D) $(dot-target).2.o
> >       $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
> > +          --gc-sections \
> It should work if --gc-sections is used only here - last LD statement.
I think we want to add it to all three.  That way the same operations 
are performed when generating the symbols in the first two rounds of 
linking.  I think that would omit symbols that may get dropped.
Regards,
Jason