Re: [PATCH 23/28] KVM: arm64: Add NVHCR_EL2 context switching

[Joey Gouly <joey.gouly@arm.com>]

On Thu, Jul 02, 2026 at 05:02:43PM +0100, Marc Zyngier wrote:
> Since NVHCR_EL2 represents the HCR_EL2 state of the EL1 guest, it
> must be dealt with in some particular way:
> 
> - for a guest in hyp context (an L1 by definition), NVHCR_EL2 directly
>   reflects HCR_EL2 as read and written by the guest itself. It must
>   therefore be eagerly synced back with the emulation code which only
>   knows about HCR_EL2. This is unconditional if NV3 is available on
>   the host.
> 
> - For an L2 guest, NVHCR_EL2 is controlled by the L1 guest, and we
>   just context switch it like any other EL1 register. Yes, EL1, as
>   that's where this thing runs from the PoV of L1. This is conditioned
>   on the guest using NV3.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>

Reviewed-by: Joey Gouly <joey.gouly@arm.com>

> ---
>  arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 11 +++++++++++
>  arch/arm64/kvm/hyp/vhe/switch.c            | 10 ++++++++--
>  2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> index a17cbe7582de9..c382848d31947 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> @@ -172,6 +172,10 @@ static inline void __sysreg_save_el1_state(struct kvm_cpu_context *ctxt)
>  
>  	if (ctxt_has_sctlr2(ctxt))
>  		ctxt_sys_reg(ctxt, SCTLR2_EL1) = read_sysreg_el1(SYS_SCTLR2);
> +
> +	/* Retrieve L2's HCR_EL2, and save it for future use */
> +	if (is_nested_nv3_ctxt(ctxt_to_vcpu(ctxt)))
> +		ctxt_sys_reg(ctxt, NVHCR_EL2) = read_sysreg_s(SYS_NVHCR_EL2);
>  }
>  
>  static inline void __sysreg_save_el2_return_state(struct kvm_cpu_context *ctxt)
> @@ -285,6 +289,13 @@ static inline void __sysreg_restore_el1_state(struct kvm_cpu_context *ctxt,
>  
>  	if (ctxt_has_sctlr2(ctxt))
>  		write_sysreg_el1(ctxt_sys_reg(ctxt, SCTLR2_EL1), SYS_SCTLR2);
> +
> +	/*
> +	 * Publish the L2 view of HCR_EL2 to the HW if L1 is using NV3.
> +	 * Otherwise, the data is already in place in the L1's own VNCR.
> +	 */
> +	if (is_nested_nv3_ctxt(ctxt_to_vcpu(ctxt)))
> +		write_sysreg_s(ctxt_sys_reg(ctxt, NVHCR_EL2), SYS_NVHCR_EL2);
>  }
>  
>  /* Read the VCPU state's PSTATE, but translate (v)EL2 to EL1. */
> diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
> index 05bcf8bf7f978..c5c06ae41b229 100644
> --- a/arch/arm64/kvm/hyp/vhe/switch.c
> +++ b/arch/arm64/kvm/hyp/vhe/switch.c
> @@ -71,7 +71,10 @@ static u64 __compute_hcr(struct kvm_vcpu *vcpu)
>  			hcr |= HCR_NV1;
>  
>  		/* Publish the guest's view of HCR_EL2 to the HW */
> -		__vcpu_assign_sys_reg(vcpu, NVHCR_EL2, __vcpu_sys_reg(vcpu, HCR_EL2));
> +		if (cpus_have_final_cap(ARM64_HAS_NV3) && vcpu_el2_e2h_is_set(vcpu))
> +			write_sysreg_s(__vcpu_sys_reg(vcpu, HCR_EL2), SYS_NVHCR_EL2);
> +		else
> +			__vcpu_assign_sys_reg(vcpu, NVHCR_EL2, __vcpu_sys_reg(vcpu, HCR_EL2));
>  
>  		/*
>  		 * Nothing in HCR_EL2 should impact running in hypervisor
> @@ -565,7 +568,10 @@ static void fixup_nv_guest_exit(struct kvm_vcpu *vcpu)
>  		*vcpu_cpsr(vcpu) |= mode;
>  
>  		/* Publish the latest HCR_EL2 to the emulation */
> -		hcr = __vcpu_sys_reg(vcpu, NVHCR_EL2);
> +		hcr = (cpus_have_final_cap(ARM64_HAS_NV3) &&
> +		       vcpu_el2_e2h_is_set(vcpu)) ?
> +			read_sysreg_s(SYS_NVHCR_EL2) :
> +			__vcpu_sys_reg(vcpu, NVHCR_EL2);
>  
>  		__vcpu_assign_sys_reg(vcpu, HCR_EL2, hcr);
>  	}
> -- 
> 2.47.3
>