Re: [Xen-devel] [PATCH v2 33/35] xen/arm32: head: Rework and document launch()

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 22 Jul 2019, Julien Grall wrote:
> Boot CPU and secondary CPUs will use different entry point to C code. At
> the moment, the decision on which entry to use is taken within launch().
> 
> In order to avoid using conditional instruction and make the call
> clearer, launch() is reworked to take in parameters the entry point and its
> arguments.
> 
> Lastly, document the behavior and the main registers usage within the
> function.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> 
> ---
>     Changes in v2:
>         - Patch added
> ---
>  xen/arch/arm/arm32/head.S | 34 ++++++++++++++++++++++++----------
>  1 file changed, 24 insertions(+), 10 deletions(-)
> 
> diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
> index e0f8c2e0cb..6d55a2119a 100644
> --- a/xen/arch/arm/arm32/head.S
> +++ b/xen/arch/arm/arm32/head.S
> @@ -170,6 +170,11 @@ primary_switched:
>          /* Use a virtual address to access the UART. */
>          mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
>  #endif
> +        PRINT("- Ready -\r\n")
> +        /* Setup the arguments for start_xen and jump to C world */
> +        mov   r0, r10                /* r0 := Physical offset */
> +        mov   r1, r8                 /* r1 := paddr(FDT) */
> +        ldr   r2, =start_xen
>          b     launch
>  ENDPROC(start)
>  
> @@ -234,6 +239,9 @@ secondary_switched:
>          /* Use a virtual address to access the UART. */
>          mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
>  #endif
> +        PRINT("- Ready -\r\n")
> +        /* Jump to C world */
> +        ldr   r2, =start_secondary
>          b     launch
>  ENDPROC(init_secondary)
>  
> @@ -578,19 +586,25 @@ setup_fixmap:
>          mov   pc, lr
>  ENDPROC(setup_fixmap)
>  
> +/*
> + * Setup the initial stack and jump to the C world
> + *
> + * Inputs:
> + *   r0 : Argument 0 of the C function to call
> + *   r1 : Argument 1 of the C function to call
> + *   r2 : C entry point
> + *
> + * Clobbers r3
> + */
>  launch:
> -        PRINT("- Ready -\r\n")
> -
> -        ldr   r0, =init_data
> -        add   r0, #INITINFO_stack    /* Find the boot-time stack */
> -        ldr   sp, [r0]
> +        ldr   r3, =init_data
> +        add   r3, #INITINFO_stack    /* Find the boot-time stack */
> +        ldr   sp, [r3]
>          add   sp, #STACK_SIZE        /* (which grows down from the top). */
>          sub   sp, #CPUINFO_sizeof    /* Make room for CPU save record */
> -        teq   r12, #0
> -        moveq r0, r10                /* Marshal args: - phys_offset */
> -        moveq r1, r8                 /*               - DTB address */
> -        beq   start_xen              /* and disappear into the land of C */
> -        b     start_secondary        /* (to the appropriate entry point) */
> +
> +        /* Jump to C world */
> +       bx    r2

Why bx?


>  ENDPROC(launch)
>  
>  /* Fail-stop */
> -- 
> 2.11.0
> 

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