Re: [PATCH v1 2/4] xen/arm: mpu: Define Xen start address for MPU systems

[Julien Grall <julien@xxxxxxx>]

Hi,

On 02/09/2024 15:48, Ayan Kumar Halder wrote:
> On 28/08/2024 15:49, Julien Grall wrote:
> > Hi,
> Hi Julien,
> > On 23/08/2024 17:31, 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 in this patch, we reuse the existing arm/platforms to store
> > > Armv8-R platforms' parameters. And `XEN_START_ADDRESS` is one
> > > kind of MPU based platform's parameters. So we define default
> > > `XEN_START_ADDRESS` in the MPU specific header file.
> > This seems a left-over from as previous work?
> Yes, this needs to be removed.
> > > We also introduce one Kconfig option for users to override the
> > > default Xen start address of selected platform, if they think
> > > the default address doesn't suit their scenarios. For this
> > > Kconfig option, we use an unaligned address "0xffffffff" as the
> > > default value to indicate that users haven't used a customized
> > > Xen start address.
> > >
> > > 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>
> > > ---
> > >   xen/arch/arm/Kconfig                  | 11 +++++++++++
> > >   xen/arch/arm/include/asm/mpu/layout.h | 25 +++++++++++++++++++++++++
> > >   2 files changed, 36 insertions(+)
> > >   create mode 100644 xen/arch/arm/include/asm/mpu/layout.h
> > >
> > > diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
> > > index 686948cefd..d722ca690c 100644
> > > --- a/xen/arch/arm/Kconfig
> > > +++ b/xen/arch/arm/Kconfig
> > > @@ -23,6 +23,17 @@ config ARCH_DEFCONFIG
> > >       default "arch/arm/configs/arm32_defconfig" if ARM_32
> > >       default "arch/arm/configs/arm64_defconfig" if ARM_64
> > >   +config XEN_START_ADDRESS
> > > +    hex "Xen start address: keep default to use platform defined 
> > > address"
> > > +    default 0xFFFFFFFF
> > > +    depends on MPU
> > > +    help
> > > +      This option is allowed to set customized address at which Xen 
> > > will be
> > > +      linked on MPU systems.
> > I don't quite understand what you mean by "allowed".
> I will rephrase this as ...
>
> "Used to set customized address at which which Xen will be linked
>
> on MPU systems. This address must be aligned to a page size.
> 0xFFFFFFFF is used as the default value to indicate that user hasn't
> customized this address."
Reading this comment, I would like to ask some clarification. In the 
context of the MPU how do you define a page size? The definition is 
pretty clear when using the MMU because the granularity if defined by 
the HW. But for the MPU, it is a bit blur. Is it still 4KB? If so, is it 
actually realistic (we don't have that many MPU regions)?
Cheers,

--
Julien Grall