linux-kernel - Re: [PATCH 10/11] KVM: arm64: nv: support SCTLR2

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Message-ID: <86pldb6xkl.wl-maz@kernel.org>
Date: Mon, 04 Aug 2025 14:11:38 +0100
From: Marc Zyngier <maz@...nel.org>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: catalin.marinas@....com,
	will@...nel.org,
	broonie@...nel.org,
	oliver.upton@...ux.dev,
	anshuman.khandual@....com,
	robh@...nel.org,
	james.morse@....com,
	mark.rutland@....com,
	joey.gouly@....com,
	ry111@...111.site,
	Dave.Martin@....com,
	ahmed.genidi@....com,
	kevin.brodsky@....com,
	scott@...amperecomputing.com,
	mbenes@...e.cz,
	james.clark@...aro.org,
	frederic@...nel.org,
	rafael@...nel.org,
	pavel@...nel.org,
	ryan.roberts@....com,
	suzuki.poulose@....com,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org,
	kvmarm@...ts.linux.dev
Subject: Re: [PATCH 10/11] KVM: arm64: nv: support SCTLR2_ELx on nv

On Mon, 04 Aug 2025 13:17:23 +0100,
Yeoreum Yun <yeoreum.yun@....com> wrote:

[...]

> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index dc1d26559bfa..a4d3b2d2fd80 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -1704,6 +1704,19 @@ int kvm_init_nv_sysregs(struct kvm_vcpu *vcpu)
>  			 TCR2_EL2_AMEC1 | TCR2_EL2_DisCH0 | TCR2_EL2_DisCH1);
>  	set_sysreg_masks(kvm, TCR2_EL2, res0, res1);
> 
> +	/*
> +	 * SCTLR2_EL2 - until explicit support for each feature, set all as RES0.
> +	 */
> +	res0 = SCTLR2_EL2_RES0 | SCTLR2_EL2_EMEC;
> +	res0 |= SCTLR2_EL2_EASE;
> +	res0 |= SCTLR2_EL2_NMEA;
> +	res0 |= (SCTLR2_EL2_EnADERR | SCTLR2_EL2_EnANERR);
> +	res0 |= SCTLR2_EL2_EnIDCP128;
> +	res0 |= (SCTLR2_EL2_CPTA | SCTLR2_EL2_CPTA0 |
> +		 SCTLR2_EL2_CPTM | SCTLR2_EL2_CPTM0);
> +	res1 = SCTLR2_EL2_RES1;
> +	set_sysreg_masks(kvm, SCTLR2_EL2, res0, res1);

This patch is obsolete, but I'd like to point out that this is not the
way we describe these things. Each bit of the register needs to be
tracked against the feature it is part of, and not blindly added to
the RES0 set. See

https://lore.kernel.org/all/20250708172532.1699409-15-oliver.upton@linux.dev/

for the equivalent change.

You should *NEVER* describe a functional bit as RESx without
considering whether the feature is exposed to the guest, irrespective
of what the kernel supports.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.