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

[Daniel Henrique Barboza <danielhb413@gmail.com>]

Alexey,

I had to amend your commit due to Gitlab CI complaining about ...

On 5/4/22 03:55, Alexey Kardashevskiy wrote:
> 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);


... this code. This is problematic when compiling in a 32 bit environment because
the definition of long (long) unsigned differs from the usual 64 bit env we use:



../hw/ppc/spapr.c: In function 'spapr_machine_init':
../hw/ppc/spapr.c:2998:25: error: format '%lx' expects argument of type 'long unsigned int', but argument 2 has type 'uint64_t' {aka 'long long unsigned int'} [-Werror=format=]
  2998 |             warn_report("spapr: kernel_addr changed from 0x%lx to 0x%lx",
       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  2999 |                         spapr->kernel_addr, loaded_addr);
       |                         ~~~~~~~~~~~~~~~~~~
       |                              |
       |                              uint64_t {aka long long unsigned int}
../hw/ppc/spapr.c:2998:25: error: format '%lx' expects argument of type 'long unsigned int', but argument 3 has type 'uint64_t' {aka 'long long unsigned int'} [-Werror=format=]
  2998 |             warn_report("spapr: kernel_addr changed from 0x%lx to 0x%lx",
       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  2999 |                         spapr->kernel_addr, loaded_addr);
       |                                             ~~~~~~~~~~~
       |                                             |
       |                                             uint64_t {aka long long unsigned int}
cc1: all warnings being treated as errors


I've fixed it by doing the following:


diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 156e799ae9..8d5bdfc20f 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -2995,7 +2995,7 @@ static void spapr_machine_init(MachineState *machine)
          }
  
          if (spapr->kernel_addr != loaded_addr) {
-            warn_report("spapr: kernel_addr changed from 0x%lx to 0x%lx",
+            warn_report("spapr: kernel_addr changed from 0x%"PRIx64
+                        " to 0x%"PRIx64,
                          spapr->kernel_addr, loaded_addr);
              spapr->kernel_addr = loaded_addr;
          }



If you're ok with this fixup we can keep it as is. Otherwise feel free to send
another version.


Thanks,


Daniel





> +            spapr->kernel_addr = loaded_addr;
> +        }
> +
>           /* load initrd */
>           if (initrd_filename) {
>               /* Try to locate the initrd in the gap between the kernel