[Xen-devel,12/20] xen/arm32: head: Always zero r3 before update a page-table entry

Message ID 20190422164937.21350-13-julien.grall@arm.com
State Superseded
Headers show
Series
  • xen/arm: Clean-up & fixes in boot/mm code
Related show

Commit Message

Julien Grall April 22, 2019, 4:49 p.m.
The boot code is using r2 and r3 to hold the page-table entry value.
While r2 is always updated before storing the value, this is not always
the case for r3.

Thankfully today, r3 will always be zero when we care. But this is
difficult to track and error-prone.

So always zero r3 within the few instructions before the write the
page-table entry.

Signed-off-by: Julien Grall <julien.grall@arm.com>
---
 xen/arch/arm/arm32/head.S | 3 +++
 1 file changed, 3 insertions(+)

Comments

Andrii Anisov May 3, 2019, 3:57 p.m. | #1
Hello Julien,

On 22.04.19 19:49, Julien Grall wrote:
> The boot code is using r2 and r3 to hold the page-table entry value.
> While r2 is always updated before storing the value, this is not always
> the case for r3.
> 
> Thankfully today, r3 will always be zero when we care. But this is
> difficult to track and error-prone.
> 
> So always zero r3 within the few instructions before the write the
> page-table entry.
> 
> Signed-off-by: Julien Grall <julien.grall@arm.com>
> ---
>   xen/arch/arm/arm32/head.S | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
> index 3448817aab..0536b62aec 100644
> --- a/xen/arch/arm/arm32/head.S
> +++ b/xen/arch/arm/arm32/head.S
> @@ -270,6 +270,7 @@ cpu_init_done:
>           orr   r2, r2, #PT_UPPER(MEM) /* r2:r3 := section map */
>           orr   r2, r2, #PT_LOWER(MEM)
>           lsl   r1, r1, #3             /* r1 := Slot offset */
> +        mov   r3, #0x0>           strd  r2, r3, [r4, r1]       /* Mapping of paddr(start) */
>           mov   r6, #1                 /* r6 := identity map now in place */
>   
> @@ -377,6 +378,7 @@ paging:
>           lsl   r2, r2, #THIRD_SHIFT   /* 4K aligned paddr of UART */
>           orr   r2, r2, #PT_UPPER(DEV_L3)
>           orr   r2, r2, #PT_LOWER(DEV_L3) /* r2:r3 := 4K dev map including UART */
> +        mov   r3, #0

What's the difference between `#0x0` and `#0`? I've seen the usage is mixed across the file, but not sure why. Could it be unified?

>           strd  r2, r3, [r1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's slot */
>   1:
>   
> @@ -388,6 +390,7 @@ paging:
>           orr   r2, r2, #PT_LOWER(PT)  /* r2:r3 := table map of xen_fixmap */
>           ldr   r4, =FIXMAP_ADDR(0)
>           mov   r4, r4, lsr #(SECOND_SHIFT - 3)   /* r4 := Slot for FIXMAP(0) */
> +        mov   r3, #0
Ditto.

>           strd  r2, r3, [r1, r4]       /* Map it in the fixmap's slot */
>   
>           /* Use a virtual address to access the UART. */
> 

With the minor comments:

Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>
Julien Grall May 3, 2019, 4:21 p.m. | #2
On 03/05/2019 16:57, Andrii Anisov wrote:
> Hello Julien,
> 
> On 22.04.19 19:49, Julien Grall wrote:
>> The boot code is using r2 and r3 to hold the page-table entry value.
>> While r2 is always updated before storing the value, this is not always
>> the case for r3.
>>
>> Thankfully today, r3 will always be zero when we care. But this is
>> difficult to track and error-prone.
>>
>> So always zero r3 within the few instructions before the write the
>> page-table entry.
>>
>> Signed-off-by: Julien Grall <julien.grall@arm.com>
>> ---
>>   xen/arch/arm/arm32/head.S | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
>> index 3448817aab..0536b62aec 100644
>> --- a/xen/arch/arm/arm32/head.S
>> +++ b/xen/arch/arm/arm32/head.S
>> @@ -270,6 +270,7 @@ cpu_init_done:
>>           orr   r2, r2, #PT_UPPER(MEM) /* r2:r3 := section map */
>>           orr   r2, r2, #PT_LOWER(MEM)
>>           lsl   r1, r1, #3             /* r1 := Slot offset */
>> +        mov   r3, #0x0>           strd  r2, r3, [r4, r1]       /* Mapping of 
>> paddr(start) */
>>           mov   r6, #1                 /* r6 := identity map now in place */
>> @@ -377,6 +378,7 @@ paging:
>>           lsl   r2, r2, #THIRD_SHIFT   /* 4K aligned paddr of UART */
>>           orr   r2, r2, #PT_UPPER(DEV_L3)
>>           orr   r2, r2, #PT_LOWER(DEV_L3) /* r2:r3 := 4K dev map including 
>> UART */
>> +        mov   r3, #0
> 
> What's the difference between `#0x0` and `#0`? I've seen the usage is mixed 
> across the file, but not sure why. Could it be unified?

No difference, matter of taste. The file seems to use 0x0 in more places, so I 
will use 0x0 in this patch as well.

> 
>>           strd  r2, r3, [r1, #(FIXMAP_CONSOLE*8)] /* Map it in the first 
>> fixmap's slot */
>>   1:
>> @@ -388,6 +390,7 @@ paging:
>>           orr   r2, r2, #PT_LOWER(PT)  /* r2:r3 := table map of xen_fixmap */
>>           ldr   r4, =FIXMAP_ADDR(0)
>>           mov   r4, r4, lsr #(SECOND_SHIFT - 3)   /* r4 := Slot for FIXMAP(0) */
>> +        mov   r3, #0
> Ditto.
> 
>>           strd  r2, r3, [r1, r4]       /* Map it in the fixmap's slot */
>>           /* Use a virtual address to access the UART. */
>>
> 
> With the minor comments:
> 
> Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>

Thank you.

Cheers,

Patch

diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
index 3448817aab..0536b62aec 100644
--- a/xen/arch/arm/arm32/head.S
+++ b/xen/arch/arm/arm32/head.S
@@ -270,6 +270,7 @@  cpu_init_done:
         orr   r2, r2, #PT_UPPER(MEM) /* r2:r3 := section map */
         orr   r2, r2, #PT_LOWER(MEM)
         lsl   r1, r1, #3             /* r1 := Slot offset */
+        mov   r3, #0x0
         strd  r2, r3, [r4, r1]       /* Mapping of paddr(start) */
         mov   r6, #1                 /* r6 := identity map now in place */
 
@@ -377,6 +378,7 @@  paging:
         lsl   r2, r2, #THIRD_SHIFT   /* 4K aligned paddr of UART */
         orr   r2, r2, #PT_UPPER(DEV_L3)
         orr   r2, r2, #PT_LOWER(DEV_L3) /* r2:r3 := 4K dev map including UART */
+        mov   r3, #0
         strd  r2, r3, [r1, #(FIXMAP_CONSOLE*8)] /* Map it in the first fixmap's slot */
 1:
 
@@ -388,6 +390,7 @@  paging:
         orr   r2, r2, #PT_LOWER(PT)  /* r2:r3 := table map of xen_fixmap */
         ldr   r4, =FIXMAP_ADDR(0)
         mov   r4, r4, lsr #(SECOND_SHIFT - 3)   /* r4 := Slot for FIXMAP(0) */
+        mov   r3, #0
         strd  r2, r3, [r1, r4]       /* Map it in the fixmap's slot */
 
         /* Use a virtual address to access the UART. */