On 2025-12-12 08:22, Jan Beulich wrote:
On 12.12.2025 02:34, Jason Andryuk wrote:
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?
I haven't looked into this in detail yet, but ...
--- 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" \
... wouldn't you need another .attach_to_group here and ...
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"\
... here? Or alternatively use the 'G' section flag to the specify the group
name?
The '?' flag puts the new section in the previous group, so it doesn't
have to be specified. I have used 'G' and %%S with similar results.
The example vpmu output above shows that is working. I can't get to
linking with --gc-sections yes to see if %%S is no longer necessary with
proper groups.
The problem is "the current function" needs to be assigned to the same
group, and that is what I hoped to address with .attach_to_group. From
what I can tell, the function-section is not assigned to a group without
.attach_to_group.
As to debug info, I wonder whether playing with groups behind the back of the
compiler is going to work well. Iirc it groups sections itself, too. Did you
look at the generated assembly with this in mind?
The generated assembly differs only by the presence of .attach_to_group
for build vs. doesn't build. Is the debug information expected to
differ according to groups? (This is all new to me). I have more to
look into, I figured I'd post what I have in case anyone had seen it before.
Thanks,
Jason
