RE: [PATCH v2 04/40] xen/arm: add an option to define Xen start address for Armv8-R

[Wei Chen <Wei.Chen@xxxxxxx>]

Hi Julien,

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
> Julien Grall
> Sent: 2023年1月18日 19:00
> To: Wei Chen <Wei.Chen@xxxxxxx>; Penny Zheng <Penny.Zheng@xxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; Bertrand Marquis
> <Bertrand.Marquis@xxxxxxx>; Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>;
> Jiamei Xie <Jiamei.Xie@xxxxxxx>
> Subject: Re: [PATCH v2 04/40] xen/arm: add an option to define Xen start
> address for Armv8-R
> 
> Hi,
> 
> On 18/01/2023 10:22, Wei Chen wrote:
> >>> Although it is unlikely that vendors using the Armv8-R IP will do so,
> it
> >>> is indeed an option. In the ID register, there are also related bits
> in
> >>> ID_AA64MMFR0_EL1 (MSA_frac) to indicate this.
> >>>
> >>>>> ---
> >>>>>     xen/arch/arm/Kconfig           |  8 ++++++++
> >>>>>     xen/arch/arm/platforms/Kconfig | 16 +++++++++++++---
> >>>>>     2 files changed, 21 insertions(+), 3 deletions(-)
> >>>>>
> >>>>> diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
> >>>>> index ace7178c9a..c6b6b612d1 100644
> >>>>> --- a/xen/arch/arm/Kconfig
> >>>>> +++ b/xen/arch/arm/Kconfig
> >>>>> @@ -145,6 +145,14 @@ config TEE
> >>>>>           This option enables generic TEE mediators support. It allows
> >>>> guests
> >>>>>           to access real TEE via one of TEE mediators implemented in
> >> XEN.
> >>>>>
> >>>>> +config XEN_START_ADDRESS
> >>>>> +       hex "Xen start address: keep default to use platform defined
> >>>> address"
> >>>>> +       default 0
> >>>>> +       depends on ARM_V8R
> >>>>
> >>>> It is still pretty unclear to me what would be the difference between
> >>>> HAS_MPU and ARM_V8R.
> >>>>
> >>>
> >>> If we don't want to support non-MPU supported Armv8-R, I think they
> are
> >> the
> >>> same. IMO, non-MPU supported Armv8-R is meaningless to Xen.
> >> OOI, why do you think this is meaningless?
> >
> > If there is Armv8-R board without EL2 MPU, how can we protect Xen?
> 
> So what you call EL2 MPU is an MPU that is following the Arm
> specification. In theory, you could have a proprietary mechanism for that.
> 
> So the question is whether a system not following the Arm specification
> is allowed.
> 

I think no, the PMSA is an architectural feature, the spec contains CPU and MPU
interfaces. Vendors can have their own hardware implementation, but need follow
the Arm spec.

But I agree that, here we could change to "depends on HAS_MPU" which will make
It easier to used by other Arm Architecture or other architecture in the future.

Cheers,
Wei Chen

> Cheers,
> 
> --
> Julien Grall