[PATCH v4] armv8: Handle EL2 Host mode

Marc Zyngier
2021-02-11T10:44:44Z

Hi Mark,

On 2021-02-11 10:22, Mark Kettenis wrote:
>> Date: Thu, 11 Feb 2021 09:58:49 +0000
>> From: Marc Zyngier <maz@kernel.org>
>>
>> On 2021-02-10 19:14, Mark Kettenis wrote:
>> > On implementations that support VHE, the layout of the CPTR_EL2
>> > register depends on whether HCR_EL2.E2H is set. If the bit is
>> > set, CPTR_EL2 uses the same layout as CPACR_EL1 and can in fact
>> > be accessed through that register. In that case, jump to the
>> > EL1 code to enable access to the FP/SIMD registers. This allows
>> > U-Boot to run on systems that pass control to U-Boot in EL2 with
>> > EL2 Host mode enabled such as machines using Apple's M1 SoC.
>> >
>> > Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
>> > ---
>> >
>> > v4: use EL1 codepath when HCR_EL2.E2H is set
>> > suggested by Marc Zyngier <maz@kernel.org>
>> >
>> > v2: rename label
>> >
>> > arch/arm/cpu/armv8/start.S | 6 ++++--
>> > 1 file changed, 4 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/arch/arm/cpu/armv8/start.S b/arch/arm/cpu/armv8/start.S
>> > index 662449156b..9e9c6140cd 100644
>> > --- a/arch/arm/cpu/armv8/start.S
>> > +++ b/arch/arm/cpu/armv8/start.S
>> > @@ -132,11 +132,13 @@ pie_fixup_done:
>> > msr cntfrq_el0, x0 /* Initialize CNTFRQ */
>> > #endif
>>
>> This seems to skip the CNTFRQ_EL0 setup. However, this should
>> probably also be done on CPUs that do not have EL3, such as M1.
>> Unless the lower level FW already deals with it? Doesn't have to
>> be part of this patch though.
>
> Hi Marc,
>
> I think the basic assumption that U-Boot makes is that if you enter in
> EL2 or EL1 there is firmware (e.g. TF-A) that does the CNTFRQ_EL0
> setup.
>
> On the M1 there is defenitely lower level FW that deals with this, so
> this assumption is still fine.

Right. As long as CNTFRQ_EL0 gets set to the correct value on all CPUs,
I'm happy!

Thanks,

M.
--
Jazz is not dead. It just smells funny...