Re: [PATCH v2 7/7] RISC-V: place .sdata / .srodata / .riscv.attributes

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 8/26/26 2:04 PM, Jan Beulich wrote:
> Of the short-data sections, only .sbss is presently mentioned in the
> linker script. Place them next to, but ahead of their "normal" data
> sections.
>
> .riscv.attributes can go towards the tail of the image, next to (ahead of)
> debug info.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Seeing where .sbss lives, does positioning really not matter at all? I
> would have expected that short-data sections want to live close together,
> and specifically close to .text / .init.text (seeing that such data is
> accessed using AUIPC). I'm puzzled that the psABI doesn't even mention
> them, hence leaving it open how exactly they are to be used.
It doesn't, and the reason is that the relevant proximity isn't to .text 
but to __global_pointer$. The small-data sections exist to let a linker 
script cluster small objects around that anchor so that ld's relaxation 
pass can fold an auipc+load pair into a single gp-relative access (-+2 
KiB window).
That pass is keyed purely on the symbol being defined 
riscv_global_pointer_value() returns 0 otherwise and the relaxation is 
skipped. We define no __global_pointer$ and head.S never loads gp (it 
appears only as a cpu_user_regs slot in entry.S), so every access stays 
the medany auipc form regardless of section.
I confirmed this by linking the same object twice (look at the script 
below, with and without the symbol: without it, zero gp-relative 
accesses; with it, the pairs collapse.
Worth noting the relaxation is section-agnostic: in the test mentioned 
below a 400-byte array in plain .bss got gp-relative too, purely because 
it landed in range. So the sections are a clustering hint, not a 
mechanism ld keys off.
The script I used:
```
mkdir -p /tmp/gp-demo && cd /tmp/gp-demo

# 1. Test code: one small variable (-> .sbss) and one large array (-> .bss)
cat > s.c <<'EOF'
int small_var;                                  /* 4 bytes   -> .sbss */
int big_arr[100];                               /* 400 bytes -> .bss  */
int read_small(void) { return small_var; }
int read_big(void)   { return big_arr[0]; }
EOF

# 2. Linker script WITHOUT __global_pointer$
cat > nogp.lds <<'EOF'
ENTRY(read_small)
SECTIONS {
  . = 0xffffffffc0000000;
  .text : { *(.text) *(.text.*) }
  .data : { *(.sdata .sdata.*) *(.data .data.*) }
  .bss  : { *(.sbss .sbss.*) *(.bss .bss.*) *(COMMON) }
  /DISCARD/ : { *(.comment) *(.note*) *(.riscv.attributes) }
}
EOF

# 3. Same script, but WITH __global_pointer$ defined
sed 's|^  \.data : {|  __global_pointer$ = . + 0x800;\n  .data : {|' 
nogp.lds > gp.lds
riscv64-linux-gnu-gcc -O2 -march=rv64ima -mabi=lp64 -mcmodel=medany \
                      -ffreestanding -c s.c -o s.o

# Check the INPUT sections: .sbss vs plain .bss (the link merges them, so
# inspect s.o, not the linked ELF)
echo "### INPUT sections the symbols live in ###"
riscv64-linux-gnu-objdump -t s.o | grep -E 'small_var|big_arr'

# Link both ways and compare the generated code
for L in nogp gp; do
  riscv64-linux-gnu-ld -T $L.lds s.o -o $L.elf 2>/dev/null
  echo "=============== $L.lds ==============="
  riscv64-linux-gnu-objdump -d --no-show-raw-insn $L.elf \
    | sed -n '/<read_small>:/,/ret/p;/<read_big>:/,/ret/p'
done

```


> What remains to eliminate orphan section warnings is the placement of
> .note.GNU-stack (which perhaps wants dealing with on all of Arm, PPC, and
> RISC-V together, ideally unifying with x86) and (odd at the first glance,
> but dealt with on x86 as well, i.e. may again want unifying) that of a
> number of .rela.* sections.
>
> --- a/xen/arch/riscv/xen.lds.S
> +++ b/xen/arch/riscv/xen.lds.S
> @@ -44,6 +44,8 @@ SECTIONS
>   
>           BUGFRAMES
>   
> +        *(.srodata)
> +        *(.srodata.*)
>           *(.rodata)
>           *(.rodata.*)
>           VPCI_ARRAY
> @@ -92,6 +94,7 @@ SECTIONS
>           SCHEDULER_ARRAY
>           HYPFS_PARAM
>   
> +        *(.sdata .sdata.*)
>           *(.data .data.*)
>           CONSTRUCTORS
>       } :text
> @@ -162,6 +165,8 @@ SECTIONS
>       /* Section for the device tree blob (if any). */
>       .dtb : { *(.dtb) } :text
>   
> +    .riscv.attributes : { *(.riscv.attributes) } :text
> +
Nit: .riscv.attributes is SHT_RISCV_ATTRIBUTES, i.e. non-alloc.
:text on it is misleading, and without an explicit address it gets 
sh_addr from .(location counter) after .dtb. Could we use matching the 
idiom used for every other non-alloc section in xen.lds.h:
  .riscv.attributes 0 : { *(.riscv.attributes) }
No functional difference either way (objcopy -O binary drops it, and I 
verified a non-alloc output section doesn't advance dot, so nothing 
downstream shifts), so purely consistency.
Is dropping orphan-handling-y := from arch/riscv/Makefile the intended 
end of this series? As if I understand correctly with such defintion we 
will miss warning so everything of that will be missed:
cd xen
riscv64-linux-gnu-ld -T arch/riscv/xen.lds prelink.o 
--orphan-handling=warn -o /tmp/t.elf 2>&1 \
  | grep 'orphan section'
/usr/bin/riscv64-linux-gnu-ld: warning: orphan section `.note.GNU-stack' 
from `prelink.o' being placed in section `.note.GNU-stack'
/usr/bin/riscv64-linux-gnu-ld: warning: orphan section `.rela.text' from 
`prelink.o' being placed in section `.rela.dyn'
/usr/bin/riscv64-linux-gnu-ld: warning: orphan section `.rela.init.text' 
from `prelink.o' being placed in section `.rela.dyn'
/usr/bin/riscv64-linux-gnu-ld: warning: orphan section 
`.rela.data.read_mostly' from `prelink.o' being placed in section 
`.rela.dyn'
/usr/bin/riscv64-linux-gnu-ld: warning: orphan section `.rela.init.data' 
from `prelink.o' being placed in section `.rela.dyn'
/usr/bin/riscv64-linux-gnu-ld: warning: orphan section 
`.rela.text.header' from `prelink.o' being placed in section `.rela.dyn'
Thanks.

~ Oleksii