Re: [PATCH 1/2] KVM: arm64: Initialize HCR_EL2.E2H early

[Leo Yan <leo.yan@arm.com>]

Hi Mark,

On Thu, Feb 27, 2025 at 06:05:25PM +0000, Mark Rutland wrote:

[...]

> +.macro init_el2_hcr	val
> +	mov_q	x0, \val
> +
> +	/*
> +	 * Compliant CPUs advertise their VHE-onlyness with
> +	 * ID_AA64MMFR4_EL1.E2H0 < 0. On such CPUs HCR_EL2.E2H is RES1, but it
> +	 * can reset into an UNKNOWN state and might not read as 1 until it has
> +	 * been initialized explicitly.

For ID_AA64MMFR4_EL1.E2H0 < 0 case, the code actually clears the
HCR_EL2.E2H bit.

Hence, the comment should be corrected as: "... it can reset into an
UNKNOWN state and might not read as 0 until it has been initialized
explicitly".

> +	 *
> +	 * Fruity CPUs seem to have HCR_EL2.E2H set to RAO/WI, but
> +	 * don't advertise it (they predate this relaxation).
> +	 *
> +	 * Initalize HCR_EL2.E2H so that later code can rely upon HCR_EL2.E2H
> +	 * indicating whether the CPU is running in E2H mode.
> +	 */

I think it is even better to clear the HCR_E2H bit first. This can
avoid any dependency on the passed parameter 'val'.

        bic     x0, x0, #HCR_E2H

> +	mrs_s	x1, SYS_ID_AA64MMFR4_EL1
> +	sbfx	x1, x1, #ID_AA64MMFR4_EL1_E2H0_SHIFT, #ID_AA64MMFR4_EL1_E2H0_WIDTH
> +	cmp	x1, #0
> +	b.lt	.LnVHE_\@
> +
> +	orr	x0, x0, #HCR_E2H
> +.LnVHE_\@:
> +	msr	hcr_el2, x0
> +	isb
> +.endm