[xen staging-4.20] xen/build: pass -fzero-init-padding-bits=all to gcc15

[patchbot@xxxxxxx]

commit 703e42cd8744afa4d21047daeac9a84e5841cc10
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Jul 7 11:33:13 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Jul 7 11:33:13 2025 +0200

    xen/build: pass -fzero-init-padding-bits=all to gcc15
    
    See the respective bullet point in the Caveats section of
    https://gcc.gnu.org/gcc-15/changes.html.
    
    While I'm unaware of us currently relying on the pre-gcc15 behavior,
    let's still play safe and retain what unknowingly we may have been
    relying upon.
    
    According to my observations, on x86 generated code changes
    - somewhere deep in modify_bars(), presumably from the struct map_data
      initializer in apply_map() (a new MOVQ),
    - in vpci_process_pending(), apparently again from the struct map_data
      initializer (and again a new MOVQ),
    - near the top of find_cpio_data(), presumably from the struct cpio_data
      initializer (a MOVW changing to a MOVQ).
    
    Requested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: 86da14178db4f40b5652e96c332d9858ca23df06
    master date: 2025-07-01 11:23:59 +0200
---
 xen/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/xen/Makefile b/xen/Makefile
index 7fa1b67487..39fd4094da 100644
--- a/xen/Makefile
+++ b/xen/Makefile
@@ -396,6 +396,7 @@ endif
 CFLAGS-$(CONFIG_CC_SPLIT_SECTIONS) += -ffunction-sections -fdata-sections
 
 CFLAGS += -nostdinc -fno-builtin -fno-common
+$(call cc-option-add,CFLAGS,CC,-fzero-init-padding-bits=all)
 CFLAGS += -Werror -Wredundant-decls -Wwrite-strings -Wno-pointer-arith
 CFLAGS += -Wdeclaration-after-statement -Wuninitialized
 $(call cc-option-add,CFLAGS,CC,-Wvla)
--
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