Re: [PATCH kvmtool v7 6/6] arm64: Handle virtio endianness reset when running nested

[Sascha Bischoff <Sascha.Bischoff@arm.com>]

On Mon, 2026-03-23 at 17:47 +0100, Andre Przywara wrote:
> From: Marc Zyngier <maz@kernel.org>
> 
> When running an EL2 guest, we need to make sure we don't sample
> SCTLR_EL1 to work out the virtio endianness, as this is likely
> to be a bit random.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>

This also looks good to me now.

Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>

Thanks,
Sascha

> ---
>  arm64/include/kvm/kvm-cpu-arch.h |  5 +--
>  arm64/kvm-cpu.c                  | 58 ++++++++++++++++++++++++++----
> --
>  2 files changed, 51 insertions(+), 12 deletions(-)
> 
> diff --git a/arm64/include/kvm/kvm-cpu-arch.h
> b/arm64/include/kvm/kvm-cpu-arch.h
> index 1af394aa..88cef813 100644
> --- a/arm64/include/kvm/kvm-cpu-arch.h
> +++ b/arm64/include/kvm/kvm-cpu-arch.h
> @@ -10,8 +10,9 @@
>  #define ARM_MPIDR_HWID_BITMASK	0xFF00FFFFFFUL
>  #define ARM_CPU_ID		3, 0, 0, 0
>  #define ARM_CPU_ID_MPIDR	5
> -#define ARM_CPU_CTRL		3, 0, 1, 0
> -#define ARM_CPU_CTRL_SCTLR_EL1	0
> +#define SYS_SCTLR_EL1		3, 0, 1, 0, 0
> +#define SYS_SCTLR_EL2		3, 4, 1, 0, 0
> +#define SYS_HCR_EL2		3, 4, 1, 1, 0
>  
>  struct kvm_cpu {
>  	pthread_t	thread;
> diff --git a/arm64/kvm-cpu.c b/arm64/kvm-cpu.c
> index 5e4f3a7d..7b012e7a 100644
> --- a/arm64/kvm-cpu.c
> +++ b/arm64/kvm-cpu.c
> @@ -12,6 +12,8 @@
>  
>  #define SCTLR_EL1_E0E_MASK	(1 << 24)
>  #define SCTLR_EL1_EE_MASK	(1 << 25)
> +#define HCR_EL2_TGE		(1UL << 27)
> +#define HCR_EL2_E2H		(1UL << 34)
>  
>  static int debug_fd;
>  
> @@ -408,7 +410,8 @@ int kvm_cpu__get_endianness(struct kvm_cpu *vcpu)
>  {
>  	struct kvm_one_reg reg;
>  	u64 psr;
> -	u64 sctlr;
> +	u64 sctlr, bit;
> +	u64 hcr = 0;
>  
>  	/*
>  	 * Quoting the definition given by Peter Maydell:
> @@ -419,8 +422,9 @@ int kvm_cpu__get_endianness(struct kvm_cpu *vcpu)
>  	 * We first check for an AArch32 guest: its endianness can
>  	 * change when using SETEND, which affects the CPSR.E bit.
>  	 *
> -	 * If we're AArch64, use SCTLR_EL1.E0E if access comes from
> -	 * EL0, and SCTLR_EL1.EE if access comes from EL1.
> +	 * If we're AArch64, determine which SCTLR register to use,
> +	 * depending on NV being used or not. Then use either the
> E0E
> +	 * bit for EL0, or the EE bit for EL1/EL2.
>  	 */
>  	reg.id = ARM64_CORE_REG(regs.pstate);
>  	reg.addr = (u64)&psr;
> @@ -430,16 +434,50 @@ int kvm_cpu__get_endianness(struct kvm_cpu
> *vcpu)
>  	if (psr & PSR_MODE32_BIT)
>  		return (psr & COMPAT_PSR_E_BIT) ? VIRTIO_ENDIAN_BE :
> VIRTIO_ENDIAN_LE;
>  
> -	reg.id = ARM64_SYS_REG(ARM_CPU_CTRL,
> ARM_CPU_CTRL_SCTLR_EL1);
> +	if (vcpu->kvm->cfg.arch.nested_virt) {
> +		reg.id = ARM64_SYS_REG(SYS_HCR_EL2);
> +		reg.addr = (u64)&hcr;
> +		if (ioctl(vcpu->vcpu_fd, KVM_GET_ONE_REG, &reg) < 0)
> +			die("KVM_GET_ONE_REG failed (HCR_EL2)");
> +	}
> +
> +	switch (psr & PSR_MODE_MASK) {
> +	case PSR_MODE_EL0t:
> +		switch (hcr & (HCR_EL2_E2H | HCR_EL2_TGE)) {
> +		case HCR_EL2_E2H | HCR_EL2_TGE: /* EL2&0 */
> +			reg.id = ARM64_SYS_REG(SYS_SCTLR_EL2);
> +			bit = SCTLR_EL1_E0E_MASK;
> +			break;
> +		case HCR_EL2_TGE:		/* EL2 */
> +			reg.id = ARM64_SYS_REG(SYS_SCTLR_EL2);
> +			bit = SCTLR_EL1_EE_MASK;
> +			break;
> +		case HCR_EL2_E2H:		/* EL1&0 (VHE) */
> +		default:			/* EL1&0 (!VHE) */
> +			reg.id = ARM64_SYS_REG(SYS_SCTLR_EL1);
> +			bit = SCTLR_EL1_E0E_MASK;
> +			break;
> +		}
> +		break;
> +	case PSR_MODE_EL1t:
> +	case PSR_MODE_EL1h:
> +		reg.id = ARM64_SYS_REG(SYS_SCTLR_EL1);
> +		bit = SCTLR_EL1_EE_MASK;
> +		break;
> +	case PSR_MODE_EL2t:
> +	case PSR_MODE_EL2h:
> +		reg.id = ARM64_SYS_REG(SYS_SCTLR_EL2);
> +		bit = SCTLR_EL1_EE_MASK;
> +		break;
> +	default:
> +		die("What's that mode???\n");
> +	}
> +
>  	reg.addr = (u64)&sctlr;
>  	if (ioctl(vcpu->vcpu_fd, KVM_GET_ONE_REG, &reg) < 0)
> -		die("KVM_GET_ONE_REG failed (SCTLR_EL1)");
> +		die("KVM_GET_ONE_REG failed (SCTLR_ELx)");
>  
> -	if ((psr & PSR_MODE_MASK) == PSR_MODE_EL0t)
> -		sctlr &= SCTLR_EL1_E0E_MASK;
> -	else
> -		sctlr &= SCTLR_EL1_EE_MASK;
> -	return sctlr ? VIRTIO_ENDIAN_BE : VIRTIO_ENDIAN_LE;
> +	return (sctlr & bit) ? VIRTIO_ENDIAN_BE : VIRTIO_ENDIAN_LE;
>  }
>  
>  void kvm_cpu__show_code(struct kvm_cpu *vcpu)