[PATCH v4 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>
---
v4:
  - Don't re-use stack pointer register (%r1) for non-stack-related purposes
    in head.S
v3: no changes.
v2:
  - Fix definition of XEN_VIRT_START macro which incorrectly used
    _AT instead of _AC.
  - Use _start instead of start as symbol referring to beginning of
    Xen binary
 xen/arch/ppc/include/asm/config.h |  2 +-
 xen/arch/ppc/ppc64/head.S         | 27 +++++++++++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/xen/arch/ppc/include/asm/config.h 
b/xen/arch/ppc/include/asm/config.h
index d060f0dca7..30438d22d2 100644
--- a/xen/arch/ppc/include/asm/config.h
+++ b/xen/arch/ppc/include/asm/config.h
@@ -39,7 +39,7 @@
     name:
 #endif

-#define XEN_VIRT_START _AT(UL, 0xc000000000000000)
+#define XEN_VIRT_START _AC(0xc000000000000000, UL)

 #define SMP_CACHE_BYTES (1 << 6)

diff --git a/xen/arch/ppc/ppc64/head.S b/xen/arch/ppc/ppc64/head.S
index 8f1e5d3ad2..a149339ad0 100644
--- a/xen/arch/ppc/ppc64/head.S
+++ b/xen/arch/ppc/ppc64/head.S
@@ -17,6 +17,33 @@ 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(%r14, _start)
+    LOAD_IMM64(%r12, XEN_VIRT_START)
+
+    /* If we're at the correct address, skip copy */
+    cmpld   %r14, %r12
+    beq     .L_correct_address
+
+    /* Copy bytes until _end */
+    LOAD_REG_ADDR(%r11, _end)
+    addi    %r14, %r14, -8
+    li      %r13, -8
+.L_copy_xen:
+    ldu     %r10, 8(%r14)
+    stdu    %r10, 8(%r13)
+    cmpld   %r14, %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