[PATCH qemu] spapr: Use address from elf parser for kernel address

[Alexey Kardashevskiy]

tl;dr: This allows Big Endian zImage booting via -kernel + x-vof=on.

QEMU loads the kernel at 0x400000 by default which works most of
the time as Linux kernels are relocatable, 64bit and compiled with "-pie"
(position independent code). This works for a little endian zImage too.

However a big endian zImage is compiled without -pie, is 32bit, linked to
0x4000000 so current QEMU ends up loading it at
0x4400000 but keeps spapr->kernel_addr unchanged so booting fails.

This uses the kernel address returned from load_elf().
If the default kernel_addr is used, there is no change in behavior (as
translate_kernel_address() takes care of this), which is:
LE/BE vmlinux and LE zImage boot, BE zImage does not.
If the VM created with "-machine kernel-addr=0,x-vof=on", then QEMU
prints a warning and BE zImage boots.

Note #1: SLOF (x-vof=off) still cannot boot a big endian zImage as
SLOF enables MSR_SF for everything loaded by QEMU and this leads to early
crash of 32bit zImage.

Note #2: BE/LE vmlinux images set MSR_SF in early boot so these just work;
a LE zImage restores MSR_SF after every CI call and we are lucky enough
not to crash before the first CI call.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---

We could probably change SLOF to always clear MSR_SF before jumping to
the kernel but this is 1) SLOF fix 2) not quite sure if it brings
lots of value.


I really wish I had this when tested this fix:
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20220406070038.3704604-1-aik@ozlabs.ru/

---
 hw/ppc/spapr.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index a4372ba1891e..89f18f6564bd 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -2971,14 +2971,16 @@ static void spapr_machine_init(MachineState *machine)
     }
 
     if (kernel_filename) {
+        uint64_t loaded_addr = 0;
+
         spapr->kernel_size = load_elf(kernel_filename, NULL,
                                       translate_kernel_address, spapr,
-                                      NULL, NULL, NULL, NULL, 1,
+                                      NULL, &loaded_addr, NULL, NULL, 1,
                                       PPC_ELF_MACHINE, 0, 0);
         if (spapr->kernel_size == ELF_LOAD_WRONG_ENDIAN) {
             spapr->kernel_size = load_elf(kernel_filename, NULL,
                                           translate_kernel_address, spapr,
-                                          NULL, NULL, NULL, NULL, 0,
+                                          NULL, &loaded_addr, NULL, NULL, 0,
                                           PPC_ELF_MACHINE, 0, 0);
             spapr->kernel_le = spapr->kernel_size > 0;
         }
@@ -2988,6 +2990,12 @@ static void spapr_machine_init(MachineState *machine)
             exit(1);
         }
 
+        if (spapr->kernel_addr != loaded_addr) {
+            warn_report("spapr: kernel_addr changed from 0x%lx to 0x%lx",
+                        spapr->kernel_addr, loaded_addr);
+            spapr->kernel_addr = loaded_addr;
+        }
+
         /* load initrd */
         if (initrd_filename) {
             /* Try to locate the initrd in the gap between the kernel
-- 
2.30.2