[PATCH 1/2] xen/arm64: correct comments

[PATCH 1/2] xen/arm64: correct comments

[Peng Fan]

The 'Base address for 4K mapping' is '(x19 >> THIRD_SHIFT) << THIRD_SHIFT'.
Also we are building 4K page mapping, not section mapping.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Julien Grall <julien.grall@arm.com>
---
 xen/arch/arm/arm64/head.S | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 946e2c9..05e3db0 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -471,10 +471,10 @@ skip_bss:
         ldr   x4, =boot_third
         add   x4, x4, x20            /* x4 := paddr (boot_third) */
 
-        lsr   x2, x19, #THIRD_SHIFT  /* Base address for 4K mapping */
-        lsl   x2, x2, #THIRD_SHIFT
-        mov   x3, #PT_MEM_L3         /* x2 := Section map */
-        orr   x2, x2, x3
+        lsr   x2, x19, #THIRD_SHIFT
+        lsl   x2, x2, #THIRD_SHIFT  /* Base address for 4K mapping */
+        mov   x3, #PT_MEM_L3
+        orr   x2, x2, x3            /* x2 := Page map */
 
         /* ... map of vaddr(start) in boot_third */
         mov   x1, xzr
-- 
2.6.2


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[PATCH 2/2] xen/arm64: use shift operator

[Peng Fan]

Use shift operator, but not muliplication.
No function change.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Julien Grall <julien.grall@arm.com>
---
 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 */
-- 
2.6.2


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

[Julien Grall]

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.

Regards,

> Signed-off-by: Peng Fan <van.freenix@gmail.com>
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Julien Grall <julien.grall@arm.com>
> ---
>   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

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

[Peng Fan]

Hi Julien,

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.

Thanks,
Peng.

>
>Regards,
>
>>Signed-off-by: Peng Fan <van.freenix@gmail.com>
>>Cc: Stefano Stabellini <sstabellini@kernel.org>
>>Cc: Julien Grall <julien.grall@arm.com>
>>---
>>  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

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

[Julien Grall]

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.

Regards,

> Thanks,
> Peng.
>
>>
>> Regards,
>>
>>> Signed-off-by: Peng Fan <van.freenix@gmail.com>
>>> Cc: Stefano Stabellini <sstabellini@kernel.org>
>>> Cc: Julien Grall <julien.grall@arm.com>
>>> ---
>>>   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
>

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

[Peng Fan]

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@gmail.com>
>>>>Cc: Stefano Stabellini <sstabellini@kernel.org>
>>>>Cc: Julien Grall <julien.grall@arm.com>
>>>>---
>>>>  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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Re: [PATCH 1/2] xen/arm64: correct comments

[Julien Grall]

Hello Peng,

I would specific the file modified in the title.

On 20/04/16 14:54, Peng Fan wrote:
> The 'Base address for 4K mapping' is '(x19 >> THIRD_SHIFT) << THIRD_SHIFT'.

The computation is a sequence of 2 instructions. I gave a look to the 
rest of the file and the comments are usually put on the first instruction.

So I would much prefer to stay consistent with the rest of the code.

> Also we are building 4K page mapping, not section mapping.

To be fair, a page is a specialized section.

> Signed-off-by: Peng Fan <van.freenix@gmail.com>
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Julien Grall <julien.grall@arm.com>
> ---
>   xen/arch/arm/arm64/head.S | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index 946e2c9..05e3db0 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -471,10 +471,10 @@ skip_bss:
>           ldr   x4, =boot_third
>           add   x4, x4, x20            /* x4 := paddr (boot_third) */
>
> -        lsr   x2, x19, #THIRD_SHIFT  /* Base address for 4K mapping */
> -        lsl   x2, x2, #THIRD_SHIFT
> -        mov   x3, #PT_MEM_L3         /* x2 := Section map */
> -        orr   x2, x2, x3
> +        lsr   x2, x19, #THIRD_SHIFT
> +        lsl   x2, x2, #THIRD_SHIFT  /* Base address for 4K mapping */
> +        mov   x3, #PT_MEM_L3
> +        orr   x2, x2, x3            /* x2 := Page map */

The section map is the combination of 2 instructions. With this change 
we don't know quickly what the purpose of "mov x3, #PT_MEM_L3".

>
>           /* ... map of vaddr(start) in boot_third */
>           mov   x1, xzr
>

Regards,

-- 
Julien Grall

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