[Xen-devel] [PATCH v2 2/3] x86/boot: Move some __high_start code and data into init sections

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Half of __high_start is strictly for the BSP and will only be run once on
boot.  To complement 'start_secondary', create 'start_bsp' and move it into
the init.text section.

The interrupt handler 'ingore_int' is patched into the BSPs IDT, but fully
replaced with real handlers early during boot.  The BSPs IDT is used by APs
until midway through start_secondary, but after the real handlers have been
installed.  Therefore, 'ignore_int' can move to init.text.  Furthermore, its
strings can move to init.rodata.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>

---
v2: Move data to .init.rodata.  Add missing @progbits.

I know start_bsp doesn't need to be a GLOBAL(), but it prevents objdump
looking like:

  ffff82d08010005b:  85 db              test   %ebx,%ebx
  ffff82d08010005d:  0f 85 ae 46 08 00  jne    ffff82d080184711 
<start_secondary>
  ffff82d080100063:  e9 08 0f 19 00     jmpq   ffff82d080290f70 <trampoline_end>

which is confusing to say the least.  An alternative would be to insert some
explicit alignment such that trampoline_end and start_bsp were no longer at
the same virtual address, but this feels worse than using GLOBAL().
---
 xen/arch/x86/boot/x86_64.S |   14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index 8f92402..7ecfad3 100644
--- a/xen/arch/x86/boot/x86_64.S
+++ b/xen/arch/x86/boot/x86_64.S
@@ -29,7 +29,11 @@
 
         test    %ebx,%ebx
         jnz     start_secondary
+        jmp     start_bsp
 
+        .section .init.text, "ax", @progbits
+
+GLOBAL(start_bsp)
         /* Initialise IDT with simple error defaults. */
         leaq    ignore_int(%rip),%rcx
         movl    %ecx,%eax
@@ -55,10 +59,6 @@
         ud2     /* Force a panic (invalid opcode). */
 
 /* This is the default interrupt handler. */
-int_msg:
-        .asciz "Unknown interrupt (cr2=%016lx)\n"
-hex_msg:
-        .asciz "    %016lx"
 ignore_int:
         SAVE_ALL
         movq    %cr2,%rsi
@@ -75,6 +75,12 @@ ignore_int:
         jnz     0b
 1:      jmp     1b
 
+        .section .init.rodata, "a", @progbits
+
+int_msg:
+        .asciz "Unknown interrupt (cr2=%016lx)\n"
+hex_msg:
+        .asciz "    %016lx"
 
 /*** DESCRIPTOR TABLES ***/
 
-- 
1.7.10.4


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