Re: [Xen-devel] [PATCH] xen/arm64: Avoid trying to map of paddr(start) as a block entry in boot_pgtable

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2014-01-09 at 16:59 +0800, Chen Baozi wrote:
> Block entries are only supported after first-level table. Since the physical
> address of 'start' is usally less than 512GB,

Thanks, but I don't believe this is universally true. I've not been to
check but I am reasonably sure that the architectural maximum phys addr
on v8 is more than 2^39.

We've already seen a processor whose RAM starts at 128GB, it's not
beyond the realm of possibility that another processor might map it
higher up.

>  this section of codes wouldn't
> be executed and therefore useless. Remove this part of codes to avoid
> potiential bug if it tries to map paddr(start) as a level-0 block entry by
> accident.

I think this bug should be fixed rather than just deleting the code.
This probably means we need boot_first to be split into two pages in
order to support both the 1:1 mapping and the final virtual mapping.

I think the fix for this can wait for 4.5 since processors with RAM
above 512GB are not something we've actually seen in reality, and they
are broken whether we remove the existing code or not so sticking with
the status quo is the lowest risk WRT accidentally breaking existing
systems.

> Thus, we need to specified that xen image should be loaded at the address less
> than 512GB.

> Signed-off-by: Chen Baozi <baozich@xxxxxxxxx>
> ---
>  xen/arch/arm/arm64/head.S | 13 +------------
>  1 file changed, 1 insertion(+), 12 deletions(-)
> 
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index 31afdd0..bebddf0 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -266,18 +266,7 @@ skip_bss:
>          orr   x2, x1, x3             /*       + rights for linear PT */
>          str   x2, [x4, #0]           /* Map it in slot 0 */
>  
> -        /* ... map of paddr(start) in boot_pgtable */
> -        lsr   x1, x19, #39           /* Offset of base paddr in boot_pgtable 
> */
> -        cbz   x1, 1f                 /* It's in slot 0, map in boot_first
> -                                      * or boot_second later on */
> -
> -        lsl   x2, x1, #39            /* Base address for 512GB mapping */
> -        mov   x3, #PT_MEM            /* x2 := Section mapping */
> -        orr   x2, x2, x3
> -        lsl   x1, x1, #3             /* x1 := Slot offset */
> -        str   x2, [x4, x1]           /* Mapping of paddr(start)*/
> -
> -1:      /* Setup boot_first: */
> +        /* Setup boot_first: */
>          ldr   x4, =boot_first        /* Next level into boot_first */
>          add   x4, x4, x20            /* x4 := paddr(boot_first) */
>  



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