Re: Xen Armv8-R booting protocol

[Ayan Kumar Halder <ayankuma@xxxxxxx>]

On 15/04/2025 13:10, Luca Fancellu wrote:
> Hi all,
Hi Luca,
> I’ve started this thread to talk about a possible Xen booting protocol when 
> running on Armv8-R,
> currently Xen uses the Linux boot protocol and some other requirements stated 
> in docs/misc/arm/booting.txt.
>
> Unfortunately we don’t have anything already available for Armv8-R, the only 
> implementation of a bootloader
> is the boot-wrapper-aarch64 which keeps the MPU off, I/D cache off.
>
> So my proposal is to add additional requirements in our 
> docs/misc/arm/booting.txt for the case where Xen is
> started on Armv8-R, under "Firmware/bootloader requirements”.
This makes sense.
> diff --git a/docs/misc/arm/booting.txt b/docs/misc/arm/booting.txt
> index 21ae74837dcc..39f029153e38 100644
> --- a/docs/misc/arm/booting.txt
> +++ b/docs/misc/arm/booting.txt
> @@ -62,6 +62,14 @@ Xen relies on some settings the firmware has to configure in 
> EL3 before starting
>   
>   * The bit SCR_EL3.HCE (resp. SCR.HCE for 32-bit ARM) must be set to 1.
>   
> +When Xen runs on Armv8-R, the highest exception level is EL2 and the only
> +available mode is secure, hence the above requirements needs to be adjusted to
> +this case:
> +
> +* Xen must be entered in S EL2 mode.
Refer ARM DDI 0568A.c ID110520 A1.3.1

Armv8-R (AArch32) only supports a single Security state, Non-secure.

I see that it differs from Armv8-R (AArch64) ARM DDI 0600A.d ID120821, A1.3

The PE is always in Secure state.

Probably you can mention both.

> +
> +* Xen must be entered with MPU off and data cache disabled (SCTLR_EL2.M bit and
> +  SCTLR_EL2.C set to 0).
>   
>   [1] linux/Documentation/arm/booting.rst
>   Latest version: 
> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/arch/arm/booting.rst
>
> Please let me know your thoughts.
A question, Do you know if u-boot (or any other bootloader) is supported 
on Armv8-R ? Because someone needs to pass the DTB address in r2/x0.
- Ayan

> Cheers,
> Luca