Re: [PATCH v19 05/14] KVM: arm64: Set PSTATE.EXLOCK when entering an exception

On Wed, Aug 12, 2026 at 08:12:04PM +0100, Mark Brown wrote:
> As per DDI 0487 R_WTXBY we need to manage PSTATE.EXLOCK when entering an
> exception, when the exception is entered from a lower EL the bit is cleared
> while if entering from the same EL it is set to GCSCR_ELx.EXLOCKEN.
> Implement this behaviour in enter_exception64().
> 
> Reviewed-by: Yuan Yao <[email protected]>
> Signed-off-by: Mark Brown <[email protected]>
> ---
>  arch/arm64/include/uapi/asm/ptrace.h |  1 +
>  arch/arm64/kvm/hyp/exception.c       | 37 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 38 insertions(+)
> 
> diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
> index 6fed93fb2536..6113b9f9771b 100644
> --- a/arch/arm64/include/uapi/asm/ptrace.h
> +++ b/arch/arm64/include/uapi/asm/ptrace.h
> @@ -56,6 +56,7 @@
>  #define PSR_C_BIT	0x20000000
>  #define PSR_Z_BIT	0x40000000
>  #define PSR_N_BIT	0x80000000
> +#define PSR_EXLOCK_BIT 0x400000000
>  
>  #define PSR_BTYPE_SHIFT		10
>  
> diff --git a/arch/arm64/kvm/hyp/exception.c b/arch/arm64/kvm/hyp/exception.c
> index bef40ddb16db..15b00e0ee1ec 100644
> --- a/arch/arm64/kvm/hyp/exception.c
> +++ b/arch/arm64/kvm/hyp/exception.c
> @@ -65,6 +65,38 @@ static void __vcpu_write_spsr_und(struct kvm_vcpu *vcpu, u64 val)
>  		vcpu->arch.ctxt.spsr_und = val;
>  }
>  
> +static unsigned long compute_exlock(struct kvm_vcpu *vcpu,
> +				    unsigned long mode,
> +				    unsigned long target_mode)
> +{
> +	u64 gcscr;
> +
> +	if (!kvm_has_gcs(kern_hyp_va(vcpu->kvm)))
> +		return 0;
> +
> +	/* GCS can't be enabled for 32 bit */
> +	if (mode & PSR_MODE32_BIT)
> +		return 0;
> +
> +	/* When taking an exception to a higher EL EXLOCK is cleared. */
> +	if ((mode | PSR_MODE_THREAD_BIT) != target_mode)
> +		return 0;

This is essentially doing an EL comparison, which is also done in
kvm_check_illegal_exlock_return() later. Would it be clearer if we
follow the EL comparison pattern there? exlock_el_from_spsr() can have
more use too.

Thanks,
Wei-Lin Chang

> +
> +	/*
> +	 * When taking an exception to the same EL EXLOCK is set to
> +	 * the effective value of GCSR_ELx.EXLOCKEN.
> +	 */
> +	if (is_hyp_ctxt(vcpu))
> +		gcscr = __vcpu_read_sys_reg(vcpu, GCSCR_EL2);
> +	else
> +		gcscr = __vcpu_read_sys_reg(vcpu, GCSCR_EL1);
> +
> +	if (gcscr & GCSCR_ELx_EXLOCKEN)
> +		return PSR_EXLOCK_BIT;
> +
> +	return 0;
> +}
> +

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