Re: [PATCH v19 03/14] KVM: arm64: Manage GCS access and registers for guests

On Wed, Aug 12, 2026 at 08:12:02PM +0100, Mark Brown wrote:
> GCS introduces a number of system registers, on systems with GCS we need
> to context switch them and expose them to VMMs to allow guests to use
> GCS.
> 
> In order to allow guests to use GCS we also need to configure
> HCRX_EL2.GCSEn, if this is not set GCS instructions will be noops and
> CHKFEAT will report GCS as disabled.
> 
> Signed-off-by: Mark Brown <[email protected]>
> ---
>  arch/arm64/include/asm/kvm_emulate.h       |  3 +++
>  arch/arm64/include/asm/kvm_host.h          | 14 ++++++++++
>  arch/arm64/include/asm/vncr_mapping.h      |  2 ++
>  arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 31 ++++++++++++++++++++++
>  arch/arm64/kvm/hyp/vhe/sysreg-sr.c         | 10 +++++++
>  arch/arm64/kvm/sys_regs.c                  | 42 ++++++++++++++++++++++++++++++
>  6 files changed, 102 insertions(+)
> 

[...]

> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index bae2c4f92ef5..1f0da8988961 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h

[...]

> @@ -1648,6 +1658,10 @@ void kvm_set_vm_id_reg(struct kvm *kvm, u32 reg, u64 val);
>  #define kvm_has_sctlr2(k)				\
>  	(kvm_has_feat((k), ID_AA64MMFR3_EL1, SCTLRX, IMP))
>  
> +#define kvm_has_gcs(k)					\
> +	(system_supports_gcs() &&			\
> +	 kvm_has_feat((k), ID_AA64PFR1_EL1, GCS, IMP))
> +
>  static inline bool kvm_arch_has_irq_bypass(void)
>  {
>  	return true;

[...]

> diff --git a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> index a17cbe7582de..053d7b3c5104 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h

[...]

> @@ -131,6 +137,17 @@ static inline bool ctxt_has_sctlr2(struct kvm_cpu_context *ctxt)
>  	return kvm_has_sctlr2(kern_hyp_va(vcpu->kvm));
>  }
>  
> +static inline bool ctxt_has_gcs(struct kvm_cpu_context *ctxt)
> +{
> +	struct kvm_vcpu *vcpu;
> +
> +	if (!cpus_have_final_cap(ARM64_HAS_GCS))
> +		return false;
> +
> +	vcpu = ctxt_to_vcpu(ctxt);
> +	return kvm_has_feat(kern_hyp_va(vcpu->kvm), ID_AA64PFR1_EL1, GCS, IMP);
> +}

This seems to duplicate some work done in kvm_has_gcs(), can we use
kvm_has_gcs() in this function?

static inline bool ctxt_has_gcs(struct kvm_cpu_context *ctxt)
{
	struct kvm_vcpu *vcpu = ctxt_to_vcpu(ctxt);

	return kvm_has_gcs(kern_hyp_va(vcpu->kvm));
}

Thanks,
Wei-Lin Chang

[...]