Re: [Xen-devel] Clang/LLVM version requirements

[Tim Deegan <tim@xxxxxxx>]

At 15:01 +0100 on 13 Sep (1347548504), Jan Beulich wrote:
> >>> On 13.09.12 at 14:21, Tim Deegan <tim@xxxxxxx> wrote:
> > Allowing BSS would just need a few extra runes (AFAICS,
> > "--set-section-flags .bss=alloc,load,contents") in the objcopy that
> > makes reloc.bin.
> >  But I'm not sure how to make sure everything is
> > rip-relative, do if that's the real concern I'm inclined to go with
> > this compile-time check and exclude .[ro]data[.*] as well.
> > We can always fix it up to allow bss and data sections if we ever
> > actually need them.
> 
> As said, I'm fine with any approach as long as it works with all
> supported tool chains. So feel free to go the route you're
> proposing.

OK.  The patch below works for me on gccs 4.1 to 4.7 and clang 3.0.
I tries gcc 3.4 but the build already fails in a few other places.
Do we really still support gcc 3.4 like the README says?

Can I have an Ack, or would you like to apply it yourself?

Tim.

# HG changeset patch
# Parent 9e46d90d0182979a7220314ca19d2525e338aa5d
x86: check for data and BSS in reloc code at compile time.

This is a more useful failure mode than hanging at boot time, and
incidentally fixes the clang/LLVM build by removing a .subsection rune.

Signed-off-by: Tim Deegan <tim@xxxxxxx>

diff -r 9e46d90d0182 xen/arch/x86/boot/build32.mk
--- a/xen/arch/x86/boot/build32.mk      Thu Sep 13 15:00:26 2012 +0100
+++ b/xen/arch/x86/boot/build32.mk      Thu Sep 13 15:04:32 2012 +0100
@@ -20,6 +20,15 @@ CFLAGS := $(filter-out -flto,$(CFLAGS))
 
 %.o: %.c
        $(CC) $(CFLAGS) -c -fpic $< -o $@
+       $(OBJDUMP) -h $@ | sed -n '/[0-9]/{s,00*,0,g;p}' |\
+               while read idx name sz rest; do \
+                       case "$$name" in \
+                       .data|.data.*|.rodata|.rodata.*|.bss|.bss.*) \
+                               test $$sz != 0 || continue; \
+                               echo "Error: non-empty $$name: 0x$$sz" >&2; \
+                               exit $$(expr $$idx + 1);; \
+                       esac; \
+               done
 
 reloc.o: $(BASEDIR)/include/asm-x86/config.h
 .PRECIOUS: %.bin %.lnk
diff -r 9e46d90d0182 xen/arch/x86/boot/reloc.c
--- a/xen/arch/x86/boot/reloc.c Thu Sep 13 15:00:26 2012 +0100
+++ b/xen/arch/x86/boot/reloc.c Thu Sep 13 15:04:32 2012 +0100
@@ -18,10 +18,7 @@ asm (
     "    call 1f                       \n"
     "1:  pop  %ebx                     \n"
     "    mov  %eax,alloc-1b(%ebx)      \n"
-    "    mov  $_end,%ecx               \n"  /* check that BSS is empty! */
-    "    sub  $__bss_start,%ecx        \n"
-    "    jz reloc                      \n"
-    "1:  jmp 1b                        \n"
+    "    jmp  reloc                    \n"
     );
 
 /* This is our data.  Because the code must be relocatable, no BSS is
@@ -30,9 +27,6 @@ asm (
 asm (
     "alloc:                            \n"
     "    .long 0                       \n"
-    "    .subsection 1                 \n"
-    "    .p2align 4, 0xcc              \n"
-    "    .subsection 0                 \n"
     );
 
 typedef unsigned int u32;

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