RE: [PATCH 1/2] armv8: aarch32: Execute 32-bit Linux for LayerScape platforms

[Stuart Yoder]

> -----Original Message-----
> From: Robin Murphy [mailto:robin.murphy@xxxxxxx]
> Sent: Friday, September 23, 2016 8:19 AM
> To: Stuart Yoder <stuart.yoder@xxxxxxx>; Alison Wang <b18965@xxxxxxxxxxxxx>; shawnguo@xxxxxxxxxx;
> kernel@xxxxxxxxxxxxxx; Fabio Estevam Estevam <fabio.estevam@xxxxxxx>; linux@xxxxxxxxxxxxxxx; linux-arm-
> kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Scott Wood <scott.wood@xxxxxxx>; Leo Li
> <leoyang.li@xxxxxxx>
> Cc: Jason Jin <jason.jin@xxxxxxx>
> Subject: Re: [PATCH 1/2] armv8: aarch32: Execute 32-bit Linux for LayerScape platforms
> 
> On 23/09/16 14:13, Stuart Yoder wrote:
> >
> >
> >> -----Original Message-----
> >> From: Robin Murphy [mailto:robin.murphy@xxxxxxx]
> >> Sent: Friday, September 23, 2016 7:17 AM
> >> To: Alison Wang <b18965@xxxxxxxxxxxxx>; shawnguo@xxxxxxxxxx; kernel@xxxxxxxxxxxxxx; Fabio Estevam
> >> Estevam <fabio.estevam@xxxxxxx>; linux@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-
> >> kernel@xxxxxxxxxxxxxxx; Scott Wood <scott.wood@xxxxxxx>; Stuart Yoder <stuart.yoder@xxxxxxx>; Leo Li
> >> <leoyang.li@xxxxxxx>
> >> Cc: Jason Jin <jason.jin@xxxxxxx>
> >> Subject: Re: [PATCH 1/2] armv8: aarch32: Execute 32-bit Linux for LayerScape platforms
> >>
> >> Hi Alison,
> >>
> >> On 23/09/16 03:19, Alison Wang wrote:
> >>> The ARMv8 architecture supports:
> >>> 1. 64-bit execution state, AArch64.
> >>> 2. 32-bit execution state, AArch32, that is compatible with previous
> >>> versions of the ARM architecture.
> >>>
> >>> LayerScape platforms are compliant with ARMv8 architecture. This patch
> >>> is to support running 32-bit Linux kernel for LayerScape platforms.
> >>>
> >>> Verified on LayerScape LS1043ARDB, LS1012ARDB, LS1046ARDB boards.
> >>>
> >>> Signed-off-by: Ebony Zhu <ebony.zhu@xxxxxxx>
> >>> Signed-off-by: Alison Wang <alison.wang@xxxxxxx>
> >>> ---
> >>>  arch/arm/Kconfig                    |  9 +++++++++
> >>>  arch/arm/mach-imx/Kconfig           | 14 ++++++++++++++
> >>>  arch/arm/mach-imx/Makefile          |  4 +++-
> >>>  arch/arm/mach-imx/mach-layerscape.c | 23 +++++++++++++++++++++++
> >>>  4 files changed, 49 insertions(+), 1 deletion(-)
> >>>  create mode 100644 arch/arm/mach-imx/mach-layerscape.c
> >>>
> >>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> >>> index f0c8068..e8d470e 100644
> >>> --- a/arch/arm/Kconfig
> >>> +++ b/arch/arm/Kconfig
> >>> @@ -294,6 +294,15 @@ config PGTABLE_LEVELS
> >>>  	default 3 if ARM_LPAE
> >>>  	default 2
> >>>
> >>> +config ARCH_AARCH32_ES_SUPPORT
> >>> +	def_bool n
> >>> +	help
> >>> +	 The ARMv8 architecture supports 64-bit execution state, AArch64
> >>> +	 and 32-bit execution state, AArch32, that is compatible with
> >>> +	 previous versions of the ARM architecture.
> >>> +
> >>> +	 Enable AArch32 execution state support for ARMv8 architecture.
> >>
> >> What's this supposed to do, exactly? I've been running 32-bit kernels on
> >> my Juno with very little issue (beyond a couple of DT tweaks, and some
> >> firmware hacks with a corresponding bit of A64 assembly tacked on the
> >> front of the zImage to switch into AArch32 state).
> >
> > Which arch/arm/mach-* platform are you using for Juno?
> 
> I don't even know! :) I just start with a multi_v7_defconfig plus a few
> extra bits (LPAE, KVM, sil24, sky2, etc.) and it works. I guess it's the
> combination of mach-vexpress and mach-virt.

Thanks.  A question about the switch into aarch32 state... our assumption
was that the kernel starts at EL2.  In this proof of concept we're doing the
switch to aarch32/EL2 in firmware.  And what I'm being told is that the
firmware aarch64 EL2 code cannot switch to aarch32 EL2 without some 
assistance from EL3 firmware.  This is leading us to invent a new
SMC call to do this.

Did you face this?

If there is such a requirement, it's something begging for standardization.
Doesn't make sense for multiple divergent approaches for switching from
aarch64/EL2 to aarch32/EL2.

Thanks,
Stuart