[PATCH v4] armv8: Handle EL2 Host mode

[Mark Kettenis]

> Date: Thu, 11 Feb 2021 09:58:49 +0000
> From: Marc Zyngier <maz at 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 at openbsd.org>
> > ---
> > 
> > v4: use EL1 codepath when HCR_EL2.E2H is set
> >     suggested by Marc Zyngier <maz at 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.

> >  	b	0f
> > -2:	set_vbar	vbar_el2, x0
> > +2:	mrs	x1, hcr_el2
> > +	tbnz	x1, #34, 1f			/* HCR_EL2.E2H */
> > +	set_vbar vbar_el2, x0
> >  	mov	x0, #0x33ff
> >  	msr	cptr_el2, x0			/* Enable FP/SIMD */
> >  	b	0f
> > -1:	set_vbar	vbar_el1, x0
> > +1:	set_vbar vbar_el1, x0
> >  	mov	x0, #3 << 20
> >  	msr	cpacr_el1, x0			/* Enable FP/SIMD */
> >  0:
> 
> This otherwise looks good.
> 
> Acked-by: Marc Zyngier <maz at kernel.org>
> 
>          M.
> -- 
> Jazz is not dead. It just smells funny...
>