On 2025-12-11 03:29, Jan Beulich wrote:
On 11.12.2025 03:47, Andrew Cooper wrote:
On 11/12/2025 1:28 am, Jason Andryuk wrote:
On 2025-12-10 11:55, Andrew Cooper wrote:
On 09/12/2025 9:47 pm, Jason Andryuk wrote:
. = ALIGN(4);
__alt_instructions = .;
- *(.altinstructions)
+ KEEP(*(.altinstructions))
__alt_instructions_end = .;
Thinking about this, what we need is for there to be a reference tied to
the source location, referencing the replacement metadata and
replacement instructions.
Looking at https://maskray.me/blog/2021-02-28-linker-garbage-collection
might be able to do this with .reloc of type none. In fact,
BFD_RELOC_NONE seems to have been invented for precisely this purpose.
This means that if and only if the source function gets included, so
does the metadata and replacement.
With Jan's -Wa,--sectname-subst changes added to CFLAGS, this seems to
work somewhat. I'm trying to make the ALTERNATIVE place relocations
against the .altinstructions.%%S and .altinstr_replacement sections:
diff --git c/xen/arch/x86/include/asm/alternative.h
w/xen/arch/x86/include/asm/alternative.h
index 18109e3dc5..e871bfc04c 100644
--- c/xen/arch/x86/include/asm/alternative.h
+++ w/xen/arch/x86/include/asm/alternative.h
@@ -90,18 +90,25 @@ extern void alternative_instructions(void);
/* alternative assembly primitive: */
#define ALTERNATIVE(oldinstr, newinstr, feature) \
OLDINSTR_1(oldinstr, 1) \
- ".pushsection .altinstructions, \"a\", @progbits\n" \
+ ".reloc ., BFD_RELOC_NONE, 567f\n" \
+ ".reloc ., BFD_RELOC_NONE, 568f\n" \
+ ".pushsection .altinstructions.%%S, \"a\", @progbits\n" \
+ "567:\n" \
ALTINSTR_ENTRY(feature, 1) \
".section .discard, \"a\", @progbits\n" \
".byte " alt_total_len "\n" /* total_len <= 255 */ \
DISCARD_ENTRY(1) \
".section .altinstr_replacement, \"ax\", @progbits\n" \
+ "568:\n" \
ALTINSTR_REPLACEMENT(newinstr, 1) \
".popsection\n"
There's already a symbol for the start of the replacement. We only need
to introduce a symbol for the metadata. Try something like this:
diff --git a/xen/arch/x86/include/asm/alternative.h
b/xen/arch/x86/include/asm/alternative.h
index 18109e3dc594..a1295ed816f5 100644
--- a/xen/arch/x86/include/asm/alternative.h
+++ b/xen/arch/x86/include/asm/alternative.h
@@ -55,6 +55,10 @@ extern void alternative_instructions(void);
#define as_max(a, b) "(("a") ^ ((("a") ^ ("b")) & -("AS_TRUE("("a") < ("b")")")))"
+#define REF(num) \
+ ".reloc ., BFD_RELOC_NONE, .LXEN%=_alt" #num "\n\t" \
+ ".reloc ., BFD_RELOC_NONE, "alt_repl_s(num) "\n\t"
Is it even worthwhile trying to further pursue this route if xen.efi can't
be built with it?
The alternative is section groups? I'm trying that, and it kinda works
sometimes, but .attach_to_group fails when .init.text is involved.
Here's an example that I think would work, I could make it to
--gc-sectrions:
group section [ 3] `.group' [.text.vpmu_do_msr] contains 5 sections:
[Index] Name
[ 43] .text.vpmu_do_msr
[ 44] .rela.text.vpmu_do_msr
[ 45] .altinstructions..text.vpmu_do_msr
[ 46] .rela.altinstructions..text.vpmu_do_msr
[ 47] .altinstr_replacement..text.vpmu_do_msr
But I don't make it that far. Other files blow up with tons of:
{standard input}:9098: Warning: dwarf line number information for
.init.text ignored
and
{standard input}:50083: Error: leb128 operand is an undefined symbol:
.LVU4040
Line 9098 of apic.s is .loc below:
"""
.section .init.text
.globl setup_boot_APIC_clock
.hidden setup_boot_APIC_clock
.type setup_boot_APIC_clock, @function
setup_boot_APIC_clock:
.LFB827:
.loc 1 1150 1 is_stmt 1 view -0
.cfi_startproc
pushq %rbp
"""
diff below. Any ideas?
Thanks,
Jason
diff --git a/xen/arch/x86/include/asm/alternative.h
b/xen/arch/x86/include/asm/alternative.h
index 18109e3dc5..8701d0e0a7 100644
--- a/xen/arch/x86/include/asm/alternative.h
+++ b/xen/arch/x86/include/asm/alternative.h
@@ -90,25 +90,31 @@ extern void alternative_instructions(void);
/* alternative assembly primitive: */
#define ALTERNATIVE(oldinstr, newinstr, feature) \
OLDINSTR_1(oldinstr, 1) \
- ".pushsection .altinstructions, \"a\", @progbits\n" \
+ ".attach_to_group %%S\n" \
+ ".pushsection .altinstructions.%%S, \"a?\", @progbits\n" \
ALTINSTR_ENTRY(feature, 1) \
- ".section .discard, \"a\", @progbits\n" \
+ ".popsection\n" \
+ ".pushsection .discard, \"a\", @progbits\n" \
".byte " alt_total_len "\n" /* total_len <= 255 */ \
DISCARD_ENTRY(1) \
- ".section .altinstr_replacement, \"ax\", @progbits\n" \
+ ".popsection\n" \
+ ".pushsection .altinstr_replacement.%%S, \"ax?\", @progbits\n"\
ALTINSTR_REPLACEMENT(newinstr, 1) \
".popsection\n"
#define ALTERNATIVE_2(oldinstr, newinstr1, feature1, newinstr2,
feature2) \
OLDINSTR_2(oldinstr, 1, 2) \
- ".pushsection .altinstructions, \"a\", @progbits\n" \
+ ".attach_to_group %%S\n" \
+ ".pushsection .altinstructions.%%S, \"a?\", @progbits\n" \
ALTINSTR_ENTRY(feature1, 1) \
ALTINSTR_ENTRY(feature2, 2) \
- ".section .discard, \"a\", @progbits\n" \
+ ".popsection\n" \
+ ".pushsection .discard, \"a\", @progbits\n" \
".byte " alt_total_len "\n" /* total_len <= 255 */ \
DISCARD_ENTRY(1) \
DISCARD_ENTRY(2) \
- ".section .altinstr_replacement, \"ax\", @progbits\n" \
+ ".popsection\n" \
+ ".pushsection .altinstr_replacement.%%S, \"ax?\", @progbits\n"\
ALTINSTR_REPLACEMENT(newinstr1, 1) \
ALTINSTR_REPLACEMENT(newinstr2, 2) \
".popsection\n"
