[PATCH v4 04/29] arm64: disable trapping of POR_EL0 to EL2

[Joey Gouly]

On Thu, Jul 25, 2024 at 04:44:13PM +0100, Dave Martin wrote:
> Hi,
> 
> On Fri, May 03, 2024 at 02:01:22PM +0100, Joey Gouly wrote:
> > Allow EL0 or EL1 to access POR_EL0 without being trapped to EL2.
> > 
> > Signed-off-by: Joey Gouly <joey.gouly at arm.com>
> > Cc: Catalin Marinas <catalin.marinas at arm.com>
> > Cc: Will Deacon <will at kernel.org>
> > Acked-by: Catalin Marinas <catalin.marinas at arm.com>
> > ---
> >  arch/arm64/include/asm/el2_setup.h | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/el2_setup.h b/arch/arm64/include/asm/el2_setup.h
> > index b7afaa026842..df5614be4b70 100644
> > --- a/arch/arm64/include/asm/el2_setup.h
> > +++ b/arch/arm64/include/asm/el2_setup.h
> > @@ -184,12 +184,20 @@
> >  .Lset_pie_fgt_\@:
> >  	mrs_s	x1, SYS_ID_AA64MMFR3_EL1
> >  	ubfx	x1, x1, #ID_AA64MMFR3_EL1_S1PIE_SHIFT, #4
> > -	cbz	x1, .Lset_fgt_\@
> > +	cbz	x1, .Lset_poe_fgt_\@
> >  
> >  	/* Disable trapping of PIR_EL1 / PIRE0_EL1 */
> >  	orr	x0, x0, #HFGxTR_EL2_nPIR_EL1
> >  	orr	x0, x0, #HFGxTR_EL2_nPIRE0_EL1
> >  
> > +.Lset_poe_fgt_\@:
> > +	mrs_s	x1, SYS_ID_AA64MMFR3_EL1
> > +	ubfx	x1, x1, #ID_AA64MMFR3_EL1_S1POE_SHIFT, #4
> > +	cbz	x1, .Lset_fgt_\@
> > +
> > +	/* Disable trapping of POR_EL0 */
> > +	orr	x0, x0, #HFGxTR_EL2_nPOR_EL0
> 
> Do I understand correctly that this is just to allow the host to access
> its own POR_EL0, before (or unless) KVM starts up?

Yup.

> 
> KVM always overrides all the EL2 trap controls while running a guest,
> right?  We don't want this bit still set when running in a guest just
> because KVM doesn't know about POE yet.

KVM currently unconditionally traps POE regs currently, this series makes that
conditional.

> 
> (Hopefully this follows naturally from the way the KVM code works, but
> my KVM-fu is a bit rusty.)
> 
> Also, what about POR_EL1?  Do we have to reset that to something sane
> (and so untrap it here), or it is sufficient if we never turn on POE
> support in the host, via TCR2_EL1.POE?

Since the host isn't using it, we don't need to reset it. It will be reset to an unknown value for guests.

In patch 7:

+	{ SYS_DESC(SYS_POR_EL1), NULL, reset_unknown, POR_EL1 },

> 
> [...]
> 
> Cheers
> ---Dave

Thanks,
Joey