diff mbox series

[v2,19/35] xen/arm32: head: Add a macro to move an immediate constant into a 32-bit register

Message ID 20190722213958.5761-20-julien.grall@arm.com (mailing list archive)
State Superseded
Headers show
Series xen/arm: Rework head.S to make it more compliant with the Arm Arm | expand

Commit Message

Julien Grall July 22, 2019, 9:39 p.m. UTC
The current boot code is using the pattern ldr rX, =... to move an
immediate constant into a 32-bit register.

This pattern implies to load the immediate constant from a literal pool,
meaning a memory access will be performed.

The memory access can be avoided by using movw/movt instructions.

A new macro is introduced to move an immediate constant into a 32-bit
register without a memory load. Follow-up patches will make use of it.

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

---
    Changes in v2:
        - Patch added
---
 xen/arch/arm/arm32/head.S | 9 +++++++++
 1 file changed, 9 insertions(+)

Comments

Stefano Stabellini July 30, 2019, 9:11 p.m. UTC | #1
On Mon, 22 Jul 2019, Julien Grall wrote:
> The current boot code is using the pattern ldr rX, =... to move an
> immediate constant into a 32-bit register.
> 
> This pattern implies to load the immediate constant from a literal pool,
> meaning a memory access will be performed.
> 
> The memory access can be avoided by using movw/movt instructions.
> 
> A new macro is introduced to move an immediate constant into a 32-bit
> register without a memory load. Follow-up patches will make use of it.
> 
> Signed-off-by: Julien Grall <julien.grall@arm.com>

That's interesting!

Acked-by: Stefano Stabellini <sstabellini@kernel.org>


> ---
>     Changes in v2:
>         - Patch added
> ---
>  xen/arch/arm/arm32/head.S | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
> index 18ded49a04..99f4af18d8 100644
> --- a/xen/arch/arm/arm32/head.S
> +++ b/xen/arch/arm/arm32/head.S
> @@ -37,6 +37,15 @@
>  #endif
>  
>  /*
> + * Move an immediate constant into a 32-bit register using movw/movt
> + * instructions.
> + */
> +.macro mov_w reg, word
> +        movw  \reg, #:lower16:\word
> +        movt  \reg, #:upper16:\word
> +.endm
> +
> +/*
>   * Common register usage in this file:
>   *   r0  -
>   *   r1  -
> -- 
> 2.11.0
>
diff mbox series

Patch

diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
index 18ded49a04..99f4af18d8 100644
--- a/xen/arch/arm/arm32/head.S
+++ b/xen/arch/arm/arm32/head.S
@@ -37,6 +37,15 @@ 
 #endif
 
 /*
+ * Move an immediate constant into a 32-bit register using movw/movt
+ * instructions.
+ */
+.macro mov_w reg, word
+        movw  \reg, #:lower16:\word
+        movt  \reg, #:upper16:\word
+.endm
+
+/*
  * Common register usage in this file:
  *   r0  -
  *   r1  -