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

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:

quoted

Allow EL0 or EL1 to access POR_EL0 without being trapped to EL2.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@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