Re: [PATCH v4 3/6] xen/arm: mpu: Define Xen start address for MPU systems

[Julien Grall <julien@xxxxxxx>]

Hi Ayan,

On 28/10/2024 12:45, Ayan Kumar Halder wrote:
> From: Wei Chen <wei.chen@xxxxxxx>
>
> On Armv8-A, Xen has a fixed virtual start address (link address too) for all
> Armv8-A platforms. In an MMU based system, Xen can map its loaded address to
> this virtual start address. So, on Armv8-A platforms, the Xen start address does
> not need to be configurable. But on Armv8-R platforms, there is no MMU to map
> loaded address to a fixed virtual address and different platforms will have very
> different address space layout. So Xen cannot use a fixed physical address on
> MPU based system and need to have it configurable.
>
> So, we introduce a Kconfig option for users to set the start address. The start
> address needs to be aligned to 4KB. We have a check for this alignment.
>
> MPU allows us to define regions which are 64 bits aligned. This restriction
> comes from the bitfields of PRBAR, PRLAR (the lower 6 bits are 0 extended to
> provide the base and limit address of a region). This means that the start
> address of Xen needs to be at least 64 bits aligned (as it will correspond to
> the start address of memory protection region).
>
> As for now Xen on MPU tries to use the same memory alignment restrictions as it
> has been for MMU. We have added a build assertion to ensure that the page size
> is 4KB. Unlike MMU where the starting virtual address is 2MB, Xen on MPU needs
> the start address to be 4KB (ie page size) aligned.
>
> In case if the user forgets to set the start address, then 0xffffffff is used
> as default. This is to trigger the error (on alignment check) and thereby prompt
> user to set the start address.
>
> Also updated config.h so that it includes mpu/layout.h when CONFIG_MPU is
> defined.
>
> Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
> Signed-off-by: Jiamei.Xie <jiamei.xie@xxxxxxx>
> Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>

Cheers,

--
Julien Grall