Re: [Xen-devel] [PATCH MM-PART2 RESEND v2 12/19] xen/arm32: head: Always zero r3 before update a page-table entry

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 14 May 2019, 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>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
>     Changes in v2:
>         - Use 0x0 instead of 0
>         - Remove a duplicate mov r3, #0
> ---
>  xen/arch/arm/arm32/head.S | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
> index 3448817aab..18ded49a04 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 */
>  
> @@ -372,11 +373,11 @@ paging:
>  
>          /* Add UART to the fixmap table */
>          ldr   r1, =xen_fixmap        /* r1 := vaddr (xen_fixmap) */
> -        mov   r3, #0
>          lsr   r2, r11, #THIRD_SHIFT
>          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, #0x0
>          strd  r2, r3, [r1, #(FIXMAP_CONSOLE*8)] /* Map it in the first 
> fixmap's slot */
>  1:
>  
> @@ -388,6 +389,7 @@ 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, #0x0
>          strd  r2, r3, [r1, r4]       /* Map it in the fixmap's slot */
>  
>          /* Use a virtual address to access the UART. */
> -- 
> 2.11.0
> 

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