[PATCH] armv8: Fix transfer list passing with KVM

[Tom Rini]

On Tue, Nov 04, 2025 at 05:04:02PM +0200, Ilias Apalodimas wrote:

> When loading U-Boot in a low memory address, the adpr instruction
> to preserve saved_args[] end up within the memory space of QEMUs
> mapped pflashes. As a result the following store pair instructions
> lead to a crash since instructions that modify more than one registers
> can't trap into KVM.
> 
> This problem can be solved with loading U-Boot with -kernel
> and defining a different text base address. But when running with
> -bios we always start from 0x0. So let's postpone the calculation
> of saved_args[] post relocation, where we will have an address that
> doesn't collide with the QEMU flashes.
> 
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> ---
>  arch/arm/cpu/armv8/start.S | 8 ++++----
>  arch/arm/lib/crt0_64.S     | 4 ++++
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/cpu/armv8/start.S b/arch/arm/cpu/armv8/start.S
> index 40c342e97e9b..ef335de16b79 100644
> --- a/arch/arm/cpu/armv8/start.S
> +++ b/arch/arm/cpu/armv8/start.S
> @@ -387,10 +387,10 @@ ENDPROC(c_runtime_cpu_setup)
>  WEAK(save_boot_params)
>  #if (IS_ENABLED(CONFIG_BLOBLIST))
>  	/* Calculate the PC-relative address of saved_args */

This comment is no longer correct. We're saving a few regs in other regs
that can't be touched until later.

> -	adrp	x9, saved_args
> -	add	x9, x9, :lo12:saved_args
> -	stp 	x0, x1, [x9]
> -	stp 	x2, x3, [x9, #16]
> +	mov     x0, x19
> +	mov     x1, x20
> +	mov     x2, x21
> +	mov     x3, x22
>  #endif
>  	b	save_boot_params_ret	/* back to my caller */
>  ENDPROC(save_boot_params)
> diff --git a/arch/arm/lib/crt0_64.S b/arch/arm/lib/crt0_64.S
> index 3e7627aa389b..d6246010f368 100644
> --- a/arch/arm/lib/crt0_64.S
> +++ b/arch/arm/lib/crt0_64.S
> @@ -132,6 +132,10 @@ ENTRY(_main)
>  	b	relocate_code
>  
>  relocation_return:
> +       adrp    x9, saved_args
> +       add     x9, x9, :lo12:saved_args
> +       stp     x19, x20, [x9]
> +       stp     x21, x22, [x9, #16]

Here is where we should comment on what's being calculated (and is it
safe to do this without BLOBLIST?).

-- 
Tom
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