Re: [PATCH early-RFC 2/5] xen/arm64: Rework the memory layout

[Julien Grall <julien@xxxxxxx>]

Hi,

On 09/03/2022 11:20, Julien Grall wrote:
> From: Julien Grall <jgrall@xxxxxxxxxx>
>
> Xen is currently not fully compliant with the Arm because it will
> switch the TTBR with the MMU on.
>
> In order to be compliant, we need to disable the MMU before
> switching the TTBR. The implication is the page-tables should
> contain an identity mapping of the code switching the TTBR.
>
> If we don't rework the memory layout, we would need to find a
> virtual address that matches a physical address and doesn't clash
> with the static virtual regions. This can be a bit tricky.
>
> On arm64, the memory layout  has plenty of unused space. In most of
> the case we expect Xen to be loaded in low memory.
>
> The memory layout is reshuffled to keep the 0th slot free. Xen will now
> be loaded at (512GB + 2MB). This requires a slight tweak of the boot
> code as XEN_VIRT_START cannot be used as an immediate.
>
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
>
> ---
>
>      TODO:
>          - I vaguely recall that one of the early platform we supported add
>            the memory starting in high memory (> 1TB). I need to check
>            whether the new layout will be fine.
>          - Update the documentation to reflect the new layout
> ---
>   xen/arch/arm/arm64/head.S         |  3 ++-
>   xen/arch/arm/include/asm/config.h | 20 ++++++++++++++------
>   xen/arch/arm/mm.c                 | 14 +++++++-------
>   3 files changed, 23 insertions(+), 14 deletions(-)
>
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index 66d862fc8137..878649280d73 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -594,7 +594,8 @@ create_page_tables:
>            * need an additional 1:1 mapping, the virtual mapping will
>            * suffice.
>            */
> -        cmp   x19, #XEN_VIRT_START
> +        ldr   x0, =XEN_VIRT_START
> +        cmp   x19, x0
>           bne   1f
>           ret
>   1:
> diff --git a/xen/arch/arm/include/asm/config.h 
> b/xen/arch/arm/include/asm/config.h
> index 5db28a8dbd56..b2f31a914103 100644
> --- a/xen/arch/arm/include/asm/config.h
> +++ b/xen/arch/arm/include/asm/config.h
> @@ -107,8 +107,20 @@
>    *  Unused
>    */
>   
> +#ifdef CONFIG_ARM_32
> +
>   #define COMMON_VIRT_START       _AT(vaddr_t, 0)
>   
> +#else
> +
> +#define SLOT0_ENTRY_BITS  39
> +#define SLOT0(slot) (_AT(vaddr_t,slot) << SLOT0_ENTRY_BITS)
> +#define SLOT0_ENTRY_SIZE  SLOT0(1)
> +
> +#define COMMON_VIRT_START       SLOT(1)
This should have been SLOT0(). I got it right in my tree but merge the 
change to the wrong patch :(.
Cheers,

--
Julien Grall