Re: [PATCH 1/2] KVM: arm64: Initialise TCR2_EL2 for nVHE mode

[Marc Zyngier]

On Fri, 04 Sep 2026 00:14:41 +0100,
Mark Brown <broonie@xxxxxxxxxx> wrote:
> 
> We currently only configure TCR2_EL2 in VHE mode, this is done in
> __finalise_el2 which only runs for VHE. While all systems with TCR2_EL2
> should have VHE support users may wish to run them in nVHE mode, for
> example in order to use protected VMs.
> 
> Determine the value to load for TCR2_EL2 in C code in a similar manner to
> TCR_EL2, further patches will configure some bits in the register. When
> resetting back to the hypervisor stub clear all bits in the register in
> case something without support for TCR2_EL2 runs later.
> 
> The only practical impact should be if we are started with a misconfigured
> TCR2_EL2.
> 
> Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_asm.h   |  1 +
>  arch/arm64/kernel/asm-offsets.c    |  1 +
>  arch/arm64/kvm/arm.c               |  5 ++++-
>  arch/arm64/kvm/hyp/nvhe/hyp-init.S | 18 ++++++++++++++++--
>  4 files changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index e5b92ac09e69..eb796436d6eb 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -208,6 +208,7 @@ extern void *__vhe_undefined_symbol;
>  struct kvm_nvhe_init_params {
>  	unsigned long mair_el2;
>  	unsigned long tcr_el2;
> +	unsigned long tcr2_el2;
>  	unsigned long tpidr_el2;
>  	unsigned long stack_hyp_va;
>  	unsigned long stack_pa;
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index 9c853ed3ceab..baffe58015d6 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -118,6 +118,7 @@ int main(void)
>    DEFINE(HOST_DATA_CONTEXT,	offsetof(struct kvm_host_data, host_ctxt));
>    DEFINE(NVHE_INIT_MAIR_EL2,	offsetof(struct kvm_nvhe_init_params, mair_el2));
>    DEFINE(NVHE_INIT_TCR_EL2,	offsetof(struct kvm_nvhe_init_params, tcr_el2));
> +  DEFINE(NVHE_INIT_TCR2_EL2,	offsetof(struct kvm_nvhe_init_params, tcr2_el2));
>    DEFINE(NVHE_INIT_TPIDR_EL2,	offsetof(struct kvm_nvhe_init_params, tpidr_el2));
>    DEFINE(NVHE_INIT_STACK_HYP_VA,	offsetof(struct kvm_nvhe_init_params, stack_hyp_va));
>    DEFINE(NVHE_INIT_PGD_PA,	offsetof(struct kvm_nvhe_init_params, pgd_pa));
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 8b080804bc90..88eb0459ad4b 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -2158,7 +2158,7 @@ static int kvm_init_vector_slots(void)
>  static void __init cpu_prepare_hyp_mode(int cpu, u32 hyp_va_bits)
>  {
>  	struct kvm_nvhe_init_params *params = per_cpu_ptr_nvhe_sym(kvm_init_params, cpu);
> -	unsigned long tcr;
> +	unsigned long tcr, tcr2;
>  
>  	/*
>  	 * Calculate the raw per-cpu offset without a translation from the
> @@ -2186,6 +2186,9 @@ static void __init cpu_prepare_hyp_mode(int cpu, u32 hyp_va_bits)
>  	tcr |= TCR_T0SZ(hyp_va_bits);
>  	params->tcr_el2 = tcr;
>  
> +	tcr2 = 0;
> +	params->tcr2_el2 = tcr2;
> +
>  	params->pgd_pa = kvm_mmu_get_httbr();
>  	if (is_protected_kvm_enabled())
>  		params->hcr_el2 = HCR_HOST_NVHE_PROTECTED_FLAGS;
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-init.S b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> index 0b3e0b28dfc7..a39de9c20d27 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> @@ -137,8 +137,13 @@ alternative_if ARM64_HAS_CNP
>  alternative_else_nop_endif
>  	msr	ttbr0_el2, x2
>  
> -	ldr	x0, [x0, #NVHE_INIT_TCR_EL2]
> -	msr	tcr_el2, x0
> +	ldr	x1, [x0, #NVHE_INIT_TCR_EL2]
> +	msr	tcr_el2, x1
> +
> +alternative_if ARM64_HAS_TCR2
> +	ldr	x1, [x0, #NVHE_INIT_TCR2_EL2]
> +	msr	REG_TCR2_EL2, x1
> +alternative_else_nop_endif
>  
>  	isb
>  
> @@ -250,6 +255,15 @@ reset:
>  	mov_q	x5, INIT_SCTLR_EL2_MMU_OFF
>  	pre_disable_mmu_workaround
>  	msr	sctlr_el2, x5
> +
> +alternative_if ARM64_HAS_TCR2
> +	/*
> +	 * Disable any features we enabled in case the next user doesn't
> +	 * have TCR2_EL2 support.
> +	 */
> +	msr	REG_TCR2_EL2, xzr
> +alternative_else_nop_endif
> +

I don't see the point of this. The MMU is off, and if the next piece
of SW can't correctly initialise the HW, that's its problem.

And with this hunk gone, the rest should be moved to the following
patch.

	M.

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