Re: [Xen-devel] [PATCH 02/17] xen/arm64: head: Don't clobber x30/lr in the macro PRINT

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 10 Jun 2019, Julien Grall wrote:
>>  The current implementation of the macro PRINT will clobber x30/lr. This
> means the user should save lr if it cares about it.

By x30/lr, do you mean x0-x3 and lr? I would prefer a clearer
expression.

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> Follow-up patches will introduce more use of PRINT in place where lr
> should be preserved. Rather than requiring all the users to preserve lr,
> the macro PRINT is modified to save and restore it.
> 
> While the comment state x3 will be clobbered, this is not the case. So
> PRINT will use x3 to preserve lr.
> 
> Lastly, take the opportunity to move the comment on top of PRINT and use
> PRINT in init_uart. Both changes will be helpful in a follow-up patch.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> ---
>  xen/arch/arm/arm64/head.S | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index c8bbdf05a6..a5147c8d80 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -78,12 +78,17 @@
>   *  x30 - lr
>   */
>  
> -/* Macro to print a string to the UART, if there is one.
> - * Clobbers x0-x3. */
>  #ifdef CONFIG_EARLY_PRINTK
> +/*
> + * Macro to print a string to the UART, if there is one.
> + *
> + * Clobbers x0 - x3
> + */
>  #define PRINT(_s)           \
> +        mov   x3, lr  ;     \
>          adr   x0, 98f ;     \
>          bl    puts    ;     \
> +        mov   lr, x3  ;     \
>          RODATA_STR(98, _s)
>  #else /* CONFIG_EARLY_PRINTK */
>  #define PRINT(s)
> @@ -630,9 +635,8 @@ init_uart:
>  #ifdef EARLY_PRINTK_INIT_UART
>          early_uart_init x23, 0
>  #endif
> -        adr   x0, 1f
> -        b     puts
> -RODATA_STR(1, "- UART enabled -\r\n")
> +        PRINT("- UART enabled -\r\n")
> +        ret
>  
>  /* Print early debug messages.
>   * x0: Nul-terminated string to print.
> -- 
> 2.11.0
> 

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