Re: [Xen-devel] [PATCH v2 18/35] xen/arm64: head: Introduce a macro to get a PC-relative address of a symbol

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 22 Jul 2019, Julien Grall wrote:
> Arm64 provides instructions to load a PC-relative address, but with some
> limitations:
>    - adr is enable to cope with +/-1MB
>    - adrp is enale to cope with +/-4GB but relative to a 4KB page
>      address
> 
> Because of that, the code requires to use 2 instructions to load any Xen
> symbol. To make the code more obvious, introducing a new macro adr_l is
> introduced.
> 
> The new macro is used to replace a couple of open-coded use in
> efi_xen_start.
> 
> The macro is copied from Linux 5.2-rc4.

I was going to ask why the strange name "adr_l", now I know why :-)

I'd suggest to name it more clearly to maybe "adr_relative"?
In any case:

Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

Typo in your address


> ---
>     Changes in v2:
>         - Patch added
> ---
>  xen/arch/arm/arm64/head.S | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index 9afd89d447..2287f3ce48 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -111,6 +111,18 @@
>  
>  #endif /* !CONFIG_EARLY_PRINTK */
>  
> +/*
> + * Pseudo-op for PC relative adr <reg>, <symbol> where <symbol> is
> + * within the range +/- 4GB of the PC.
> + *
> + * @dst: destination register (64 bit wide)
> + * @sym: name of the symbol
> + */
> +.macro  adr_l, dst, sym
> +        adrp \dst, \sym
> +        add  \dst, \dst, :lo12:\sym
> +.endm
> +
>  /* Load the physical address of a symbol into xb */
>  .macro load_paddr xb, sym
>          ldr \xb, =\sym
> @@ -886,11 +898,9 @@ ENTRY(efi_xen_start)
>           * Flush dcache covering current runtime addresses
>           * of xen text/data. Then flush all of icache.
>           */
> -        adrp  x1, _start
> -        add   x1, x1, #:lo12:_start
> +        adr_l x1, _start
>          mov   x0, x1
> -        adrp  x2, _end
> -        add   x2, x2, #:lo12:_end
> +        adr_l x2, _end
>          sub   x1, x2, x1
>  
>          bl    __flush_dcache_area
> -- 
> 2.11.0
> 

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