[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH 2/2] xen/arm64: use shift operator

On 21/04/16 02:06, Peng Fan wrote:
Hi Julien,

Hello Peng,

On Wed, Apr 20, 2016 at 03:44:09PM +0100, Julien Grall wrote:
Hello Peng,

On 20/04/16 14:54, Peng Fan wrote:
Use shift operator, but not muliplication.
No function change.

Why? The compiler will calculate the address at compilation time.

Yeah. The compiler will do this. I just think in asm, we rarely use
multiplication. In this file, there is "cmp   x1, #(LPAE_ENTRIES<<3)",
here use shift operator but not multiplication.

It took me a bit of time to understand that 8 is the number of byte of an LPAE entries. IMHO the "<< 3" would be more confusing.

So I would introduce a macro to get the slot based of an index. Something like:

LPAE_SLOT(slot) ((slot) * 8)

And add a comment to explain why 8. You could even introduce LPAE_ENTRY_SIZE.




Signed-off-by: Peng Fan <van.freenix@xxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>
  xen/arch/arm/arm64/head.S | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 05e3db0..ad6e593 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -524,7 +524,7 @@ paging:
          lsl   x2, x2, #THIRD_SHIFT   /* 4K aligned paddr of UART */
          mov   x3, #PT_DEV_L3
          orr   x2, x2, x3             /* x2 := 4K dev map including UART */
-        str   x2, [x1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's 
slot */
+        str   x2, [x1, #(FIXMAP_CONSOLE << 3)] /* Map it in the first fixmap's 
slot */

          /* Map fixmap into boot_second */

Julien Grall

Julien Grall

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.