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

[Julien Grall <julien@xxxxxxx>]

On 06/05/2025 13:24, Luca Fancellu wrote:
> Hi Julien,
Hi Luca,

> > On 6 May 2025, at 12:44, Julien Grall <julien@xxxxxxx> wrote:
> >
> >
> >
> > On 29/04/2025 16:20, 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.
> > I think it would be better to update the line "Xen must be entered in NS EL2 
> > mode" to clarify the state for 64-bit Arm.
> >
> > > +> +* Xen must be entered with MPU off and data cache disabled (SCTLR_EL2.M bit 
> > > and
> > > +  SCTLR_EL2.C set to 0).
> > This line is valid for Armv8-A/Armv7-A when using the Image/zImage protocol.
> >
> > >     [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
> Just to be sure to be on the same page, are you suggesting these changes on the 
> original file?
Yes with one tweak.

> > diff --git a/docs/misc/arm/booting.txt b/docs/misc/arm/booting.txt
> index 21ae74837dcc..c00c651805d7 100644
> --- a/docs/misc/arm/booting.txt
> +++ b/docs/misc/arm/booting.txt
> @@ -58,10 +58,14 @@ Firmware/bootloader requirements
>   
>   Xen relies on some settings the firmware has to configure in EL3 before starting Xen.
I think you want to update this sentence to remove the reference to EL3. 
Even on A-profile EL3 is not mandatory (I vaguely remember one of the 
platform I worked on had no EL3).
>   
> -* Xen must be entered in NS EL2 mode
> +* Xen must be entered in:
> +  * Non-Secure EL2 mode for Armv8-A Arm64 and Arm32, Armv8-R Arm32.
> +  * Secure EL2 mode for Armv8-R Arm64.
>   
>   * The bit SCR_EL3.HCE (resp. SCR.HCE for 32-bit ARM) must be set to 1.
And then here: "When EL3 is supported, ...". This would also cover the 
R-profile change.
Cheers,

>   
> +* Xen must be entered with MMU/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
>
> Cheers,
> Luca
--
Julien Grall