[PATCH] armv8: Handle EL2 Host mode

[Mark Kettenis]

> Date: Sat, 30 Jan 2021 13:41:34 +0000
> From: Andre Przywara <andre.przywara at arm.com>
> 
> On Sat, 30 Jan 2021 13:12:32 +0100
> Mark Kettenis <kettenis at openbsd.org> wrote:
> 
> > On implementations that support VHE, the layout of the CPTR_EL2
> > register depends on whether HCR_EL2.E2H is set.  Check this bit
> > and and set the aprropriate bits 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
> > machine with Apple's M1 SoC.
> 
> That looks correct, I compared the bits against the manual.
> TCR_EL2 and CNTHCTL_EL2 are also different, have you checked that
> U-Boot's usage of it is compatible with E2H=1?

I did now:

TCR_EL2 looks ok; U-Boot will set TCR_EL2.EPD1 but since U-Boot only
uses TTBR0_ELx, this should be fine.

CNTHCTL_EL2 is only used when CONFIG_ARMV8_SWITCH_TO_EL1 is set, which
is generally not what you want, and defenitely not relevant to my use
case. (Which is using U-Boot as an EFI implementation such that I can
run OpenBSD).

I have some further diffs that allow me to boot up to a framebuffer
console on the Apple M1 Mac mini.  But I need to add some actually
hardware driver support before it makes sense to integrate those.

> And we should probably allow SVE for non-VHE systems as well, but this
> is probably another patch.
> 
> One nit below.
> 
> > 
> > Signed-off-by: Mark Kettenis <kettenis at openbsd.org>
> > ---
> >  arch/arm/cpu/armv8/start.S | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/arch/arm/cpu/armv8/start.S b/arch/arm/cpu/armv8/start.S
> > index 662449156b..a259717e67 100644
> > --- a/arch/arm/cpu/armv8/start.S
> > +++ b/arch/arm/cpu/armv8/start.S
> > @@ -133,9 +133,15 @@ pie_fixup_done:
> >  #endif
> >  	b	0f
> >  2:	set_vbar	vbar_el2, x0
> > +	mrs	x0, hcr_el2
> > +	tbnz	x0, #34, el2_host		/* HCR_EL2.E2H */
> 
> el2_host sounds a bit ambiguous, can we use el2_vhe instead? That would
> also point to the term "VHE", which is more known (and searchable)
> than the "E2H" bit.

No problem.  I'll roll a v2 later today.

Thanks,

Mark