Re: [Xen-devel] [PATCH] xen/arm64: fix section shift when mapping 2MB block in boot page table

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-01-14 at 01:06 +0800, Chen Baozi wrote:
> Section shift for level-2 page table should be #21 rather than #20.

Thanks, are you finding these issues on actual hardware or just by
inspection?

I think this change is correct, also arm32 has:

        1:      /* Setup boot_second: */
                ldr   r4, =boot_second
                add   r4, r4, r10            /* r1 := paddr (boot_second) */
        
                lsr   r2, r9, #20            /* Base address for 2MB mapping */
                lsl   r2, r2, #20
                orr   r2, r2, #PT_UPPER(MEM) /* r2:r3 := section map */
                orr   r2, r2, #PT_LOWER(MEM)
        
Either they are both wrong or both right (I think both wrong).

The effect of this error is that bit #20 of paddr(boot_second) is
preserved as bit #20 of the PTE, rather than being cleared. bit #20 of
an entry at this level is UNK/SBZP -- so we survive this mistake even if
paddr(boot_second) happens to have bit #20 set.

Really this code should use {FIRST,SECOND,THIRD}_SHIFT, and this file
already includes asm/page.h so they should be available...

> 
> Signed-off-by: Chen Baozi <baozich@xxxxxxxxx>
> ---
>  xen/arch/arm/arm64/head.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index bebddf0..ad35f60 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -292,8 +292,8 @@ skip_bss:
>          ldr   x4, =boot_second
>          add   x4, x4, x20            /* x4 := paddr (boot_second) */
>  
> -        lsr   x2, x19, #20           /* Base address for 2MB mapping */
> -        lsl   x2, x2, #20
> +        lsr   x2, x19, #21           /* Base address for 2MB mapping */
> +        lsl   x2, x2, #21
>          mov   x3, #PT_MEM            /* x2 := Section map */
>          orr   x2, x2, x3
>  



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel