[PATCH v5 26/26] xen/riscv: do a 4th linking pass if necessary

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

Embedding the symbol table can shift sections and flip relaxation
decisions, changing code size and thus the set of emitted symbols
(e.g. gap end markers).  Re-link with a regenerated table when its
size differs from the previous pass.

This mirrors commit 35de7285d508 ("Arm: do a 4th linking pass if
necessary"), which riscv didn't receive when the underlying
pass-2-vs-pass-3 check was extended to it in commit aa786d6e3467
("non-x86/symbols: check table sizes don't change between linking
passes 2 and 3").

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
I faced this issue again in downstream:
  https://gitlab.com/xen-project/people/olkur/xen/-/jobs/15171254706

It was found that the difference between .xen-syms.0 and .xen-syms.1 is in
alignment(?) gap between turn_on_mmu() and the end of simple_strtoull.
Specifically 0xffffffffc00c05b0 is present in pass 1 but not present in
pass 2 (where end of simple_strtoull() is just equal to ffffffffc00c05c0)
because code was shifted:

```
.xen-syms.0:     file format elf64-littleriscv

...

ffffffffc00c0534:       f2e794e3                bne     a5,a4,ffffffffc00c045c 
<simple_strtoull+0x18>
ffffffffc00c0538:       00280813                addi    a6,a6,2
ffffffffc00c053c:       f21ff06f                j       ffffffffc00c045c 
<simple_strtoull+0x18>
ffffffffc00c0540:       00068813                mv      a6,a3
ffffffffc00c0544:       00800613                li      a2,8
ffffffffc00c0548:       f15ff06f                j       ffffffffc00c045c 
<simple_strtoull+0x18>
ffffffffc00c054c:       00277713                andi    a4,a4,2
ffffffffc00c0550:       04070463                beqz    a4,ffffffffc00c0598 
<simple_strtoull+0x154>
ffffffffc00c0554:       fe06869b                addiw   a3,a3,-32
ffffffffc00c0558:       0ff6f693                zext.b  a3,a3
ffffffffc00c055c:       fc96879b                addiw   a5,a3,-55
ffffffffc00c0560:       04c7f263                bgeu    a5,a2,ffffffffc00c05a4 
<simple_strtoull+0x160>
ffffffffc00c0564:       02a60533                mul     a0,a2,a0
ffffffffc00c0568:       00f50533                add     a0,a0,a5
ffffffffc00c056c:       00180813                addi    a6,a6,1
ffffffffc00c0570:       00084683                lbu     a3,0(a6)
ffffffffc00c0574:       0006879b                sext.w  a5,a3
ffffffffc00c0578:       00d30733                add     a4,t1,a3
ffffffffc00c057c:       00074703                lbu     a4,0(a4)
ffffffffc00c0580:       04477893                andi    a7,a4,68
ffffffffc00c0584:       02088063                beqz    a7,ffffffffc00c05a4 
<simple_strtoull+0x160>
ffffffffc00c0588:       00477893                andi    a7,a4,4
ffffffffc00c058c:       fc0880e3                beqz    a7,ffffffffc00c054c 
<simple_strtoull+0x108>
ffffffffc00c0590:       fd07879b                addiw   a5,a5,-48
ffffffffc00c0594:       fcdff06f                j       ffffffffc00c0560 
<simple_strtoull+0x11c>
ffffffffc00c0598:       fc97879b                addiw   a5,a5,-55
ffffffffc00c059c:       fc5ff06f                j       ffffffffc00c0560 
<simple_strtoull+0x11c>
ffffffffc00c05a0:       00000513                li      a0,0
ffffffffc00c05a4:       00058463                beqz    a1,ffffffffc00c05ac 
<simple_strtoull+0x168>
ffffffffc00c05a8:       0105b023                sd      a6,0(a1)
ffffffffc00c05ac:       00008067                ret
        ...

ffffffffc00c05c0 <turn_on_mmu>:
ffffffffc00c05c0:       12000073                sfence.vma
ffffffffc00c05c4:       00800293                li      t0,8
ffffffffc00c05c8:       03c29293                slli    t0,t0,0x3c
ffffffffc00c05cc:       000fc317                auipc   t1,0xfc
ffffffffc00c05d0:       a3430313                addi    t1,t1,-1484 # 
ffffffffc01bc000 <stage1_pgtbl_root>
ffffffffc00c05d4:       00c35313                srli    t1,t1,0xc
ffffffffc00c05d8:       00536333                or      t1,t1,t0
ffffffffc00c05dc:       18031073                csrw    satp,t1
ffffffffc00c05e0:       00050067                jr      a0

ffffffffc00c05e4 <_ident_end>:
ffffffffc00c05e4:       0000                    .insn   2, 0x0000
        ...
```

So the tool (symbols.c) emits an entry at start+size only when a gap
follows; the gap closed in pass 2.  (look at the code of want_symbol_end()
in symbols.c) what leads to difference in one entry in symbols table between
passes.
---
 xen/arch/riscv/Makefile | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/xen/arch/riscv/Makefile b/xen/arch/riscv/Makefile
index fc6b34661111..046f73f4d87c 100644
--- a/xen/arch/riscv/Makefile
+++ b/xen/arch/riscv/Makefile
@@ -50,9 +50,21 @@ $(TARGET)-syms: $(objtree)/prelink.o $(obj)/xen.lds
                | $(objtree)/tools/symbols $(all_symbols) --sysv --sort \
                > $(dot-target).2.S
        $(MAKE) $(build)=$(@D) $(dot-target).2.o
-       $(call compare-symbol-tables, $(dot-target).1.o, $(dot-target).2.o)
+       if ! { $(call compare-symbol-tables, $(dot-target).1.o, 
$(dot-target).2.o) >/dev/null; }; \
+       then \
+               set -e; \
+               $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
+                   $(dot-target).2.o -o $(dot-target).2; \
+               $(NM) -pa --format=sysv $(dot-target).2 \
+                       | $(objtree)/tools/symbols $(all_symbols) --sysv --sort 
\
+                       > $(dot-target).3.S; \
+               $(MAKE) $(build)=$(@D) $(dot-target).3.o; \
+               $(call compare-symbol-tables, $(dot-target).2.o, 
$(dot-target).3.o); \
+       else \
+               ln -sf $(dot-target).2.o $(dot-target).3.o; \
+       fi
        $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
-           $(dot-target).2.o -o $@
+           $(dot-target).3.o -o $@
        $(NM) -pa --format=sysv $@ \
                | $(objtree)/tools/symbols --all-symbols --xensyms --sysv 
--sort \
                > $@.map
-- 
2.54.0