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

[Peng Fan <van.freenix@xxxxxxxxx>]

Hi Julien,
On Fri, Apr 22, 2016 at 05:08:26PM +0100, Julien Grall wrote:
>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.

Thanks for comments. I'll try to submit a new patch following your suggestion.

Thanks,
Peng.

>
>Regards,
>
>>Thanks,
>>Peng.
>>
>>>
>>>Regards,
>>>
>>>>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 */
>>>>  1:
>>>>
>>>>          /* Map fixmap into boot_second */
>>>>
>>>
>>>--
>>>Julien Grall
>>
>
>-- 
>Julien Grall

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