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

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

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@xxxxxxx>
> ---
>   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@xxxxxxxx>

--
Sincerely,
Andrii Anisov.

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