Re: [PATCH] xen/arm: skip first 32 bytes of zimage32

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

+xen-devel

On Sat, 19 Mar 2022, Stefano Stabellini wrote:
> From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> 
> The first 32 bytes of zImage32 are NOPs, not useful just there for
> compatibility. The reason is that some bootloaders skip the first 32
> bytes when starting the kernel. See the comment in Linux
> arch/arm/boot/compressed/head.S.
> 
> Since the introduction of CONFIG_EFI in Linux arm32, those NOPs
> operations have changed implementation from:
> 
>     mov r0, r0
> 
> to:
>     .inst   MZ_MAGIC | (0x1310 << 16)   @ tstne r0, #0x4d000
> 
> See arch/arm/boot/compressed/efi-header.S.
> 
> The new implementation doesn't work on Xen (at least on all versions of
> QEMU I tried).
> 
> Since the first 32 bytes are made to be skipped anyway, skip them. This
> enables Xen to load and start successfully (on QEMU) aarch32 kernels
> with CONFIG_EFI.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> ---
>  xen/arch/arm/kernel.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/arm/kernel.c b/xen/arch/arm/kernel.c
> index 8f43caa186..105a010bf4 100644
> --- a/xen/arch/arm/kernel.c
> +++ b/xen/arch/arm/kernel.c
> @@ -411,10 +411,10 @@ static int __init kernel_zimage32_probe(struct 
> kernel_info *info,
>          }
>      }
>  
> -    info->zimage.kernel_addr = addr;
> +    info->zimage.kernel_addr = addr + 32;
>  
>      info->zimage.start = start;
> -    info->zimage.len = end - start;
> +    info->zimage.len = end - start - 32;
>  
>      info->load = kernel_zimage_load;
>  
> -- 
> 2.25.1
>