[PATCH 2/3] xen/ppc: Relocate kernel to physical address 0 on boot

[Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>]

Introduce a small assembly loop in `start` to copy the kernel to
physical address 0 before continuing. This ensures that the physical
address lines up with XEN_VIRT_START (0xc000000000000000) and allows us
to identity map the kernel when the MMU is set up in the next patch.

We are also able to start execution at XEN_VIRT_START after the copy
since hardware will ignore the top 4 address bits when operating in Real
Mode (MMU off).

Signed-off-by: Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>
---
 xen/arch/ppc/ppc64/head.S | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/xen/arch/ppc/ppc64/head.S b/xen/arch/ppc/ppc64/head.S
index 5ac2dad2ee..beff8257fa 100644
--- a/xen/arch/ppc/ppc64/head.S
+++ b/xen/arch/ppc/ppc64/head.S
@@ -18,6 +18,33 @@ ENTRY(start)
     addis   %r2, %r12, .TOC.-1b@ha
     addi    %r2, %r2, .TOC.-1b@l
 
+    /*
+     * Copy Xen to physical address zero and jump to XEN_VIRT_START
+     * (0xc000000000000000). This works because the hardware will ignore the 
top
+     * four address bits when the MMU is off.
+     */
+    LOAD_REG_ADDR(%r1, start)
+    LOAD_IMM64(%r12, XEN_VIRT_START)
+
+    /* If we're at the correct address, skip copy */
+    cmpld   %r1, %r12
+    beq     .L_correct_address
+
+    /* Copy bytes until _end */
+    LOAD_REG_ADDR(%r11, _end)
+    addi    %r1, %r1, -8
+    li      %r13, -8
+.L_copy_xen:
+    ldu     %r10, 8(%r1)
+    stdu    %r10, 8(%r13)
+    cmpld   %r1, %r11
+    blt     .L_copy_xen
+
+    /* Jump to XEN_VIRT_START */
+    mtctr   %r12
+    bctr
+.L_correct_address:
+
     /* set up the initial stack */
     LOAD_REG_ADDR(%r1, cpu0_boot_stack)
     li      %r11, 0
-- 
2.30.2