Re: [PATCH v4 1/7] docs/arm: Document Xen booting protocol on Armv8-R

["Orzel, Michal" <michal.orzel@xxxxxxx>]

On 30/04/2025 08:58, Luca Fancellu wrote:
> Hi Michal,
> 
>> On 30 Apr 2025, at 07:37, Orzel, Michal <Michal.Orzel@xxxxxxx> wrote:
>>
>>
>>
>> On 29/04/2025 17:20, Luca Fancellu wrote:
>>> Document the requirement needed to boot Xen on Armv8-R platforms.
>>>
>>> Signed-off-by: Luca Fancellu <luca.fancellu@xxxxxxx>
>>> ---
>>> v4 changes:
>>> - New patch
>>> ---
>>> docs/misc/arm/booting.txt | 8 ++++++++
>>> 1 file changed, 8 insertions(+)
>>>
>>> diff --git a/docs/misc/arm/booting.txt b/docs/misc/arm/booting.txt
>>> index 21ae74837dcc..719af74f1e69 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 state is secure (S) on Arm64 and non secure (NS) on Arm32, hence 
>>> the
>>> +above requirements need to be adjusted to this case:
>>> +
>>> +* Xen must be entered in S EL2 mode on Arm64 and in NS EL2 mode on Arm32.
>>> +
>>> +* 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
>>
>> Is the goal of this patch to only write firmware/bootloader requirements or
>> accommodate booting protocol in general? Asking because in this file there 
>> is a
>> requirement that Xen should be loaded below 10TB which afaict does not fit 
>> Armv8-R.
> 
> Yes you are right, but I think this still holds since we support 1TB on 
> Armv8-R at the moment?
No, this value denotes max RAM size you support and not where Xen must be
loaded. On ARMv8-A, we support max 2TB of RAM but require Xen to be loaded
anywhere up to 10TB. Do we have the same limitation on ARMv8-R?

~Michal