Re: [PATCH v10 09/30] KVM: arm64: Define internal features for SME

[Jean-Philippe Brucker <jpb@kernel.org>]

On Fri, Mar 06, 2026 at 05:01:01PM +0000, Mark Brown wrote:
> In order to simplify interdependencies in the rest of the series define
> the feature detection for SME and its subfeatures.  Due to the need for
> vector length configuration we define a flag for SME like for SVE.  We
> also have two subfeatures which add architectural state, FA64 and SME2,
> which are configured via the normal ID register scheme.
> 
> Also provide helpers which check if the vCPU is in streaming mode or has
> ZA enabled.
> 
> Reviewed-by: Fuad Tabba <tabba@google.com>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  arch/arm64/include/asm/kvm_host.h | 35 ++++++++++++++++++++++++++++++++++-
>  arch/arm64/kvm/sys_regs.c         |  2 +-
>  2 files changed, 35 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 656464179ba8..906dbefc5b33 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -353,6 +353,8 @@ struct kvm_arch {
>  #define KVM_ARCH_FLAG_WRITABLE_IMP_ID_REGS		10
>  	/* Unhandled SEAs are taken to userspace */
>  #define KVM_ARCH_FLAG_EXIT_SEA				11
> +	/* SME exposed to guest */
> +#define KVM_ARCH_FLAG_GUEST_HAS_SME			12
>  	unsigned long flags;
>  
>  	/* VM-wide vCPU feature set */
> @@ -1086,7 +1088,16 @@ struct kvm_vcpu_arch {
>  #define vcpu_has_sve(vcpu)	kvm_has_sve((vcpu)->kvm)
>  #endif
>  
> -#define vcpu_has_vec(vcpu) vcpu_has_sve(vcpu)
> +#define kvm_has_sme(kvm)	(system_supports_sme() &&		\
> +				 test_bit(KVM_ARCH_FLAG_GUEST_HAS_SME, &(kvm)->arch.flags))

Can the GUEST_HAS_SME flag ever be set if !system_supports_sme()?  Seems
like it depends on KVM_ARM_VCPU_SME feature which can't be set if the
system doesn't support it, so the system_supports_sme() check is redundant

> +
> +#ifdef __KVM_NVHE_HYPERVISOR__
> +#define vcpu_has_sme(vcpu)	kvm_has_sme(kern_hyp_va((vcpu)->kvm))
> +#else
> +#define vcpu_has_sme(vcpu)	kvm_has_sme((vcpu)->kvm)
> +#endif
> +
> +#define vcpu_has_vec(vcpu) (vcpu_has_sve(vcpu) || vcpu_has_sme(vcpu))
>  
>  #ifdef CONFIG_ARM64_PTR_AUTH
>  #define vcpu_has_ptrauth(vcpu)						\
> @@ -1627,6 +1638,28 @@ 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_fa64(k)					\
> +	(system_supports_fa64() &&			\
> +	 kvm_has_feat((k), ID_AA64SMFR0_EL1, FA64, IMP))
> +
> +#define kvm_has_sme2(k)					\
> +	(system_supports_sme2() &&			\
> +	 kvm_has_feat((k), ID_AA64PFR1_EL1, SME, SME2))

Similarly it looks like KVM already prevents setting these features if the
cpufeatures value doesn't have them, so the system_supports* checks are
redundant?

Thanks,
Jean

> +
> +#ifdef __KVM_NVHE_HYPERVISOR__
> +#define vcpu_has_sme2(vcpu)	kvm_has_sme2(kern_hyp_va((vcpu)->kvm))
> +#define vcpu_has_fa64(vcpu)	kvm_has_fa64(kern_hyp_va((vcpu)->kvm))
> +#else
> +#define vcpu_has_sme2(vcpu)	kvm_has_sme2((vcpu)->kvm)
> +#define vcpu_has_fa64(vcpu)	kvm_has_fa64((vcpu)->kvm)
> +#endif
> +
> +#define vcpu_in_streaming_mode(vcpu) \
> +	(__vcpu_sys_reg(vcpu, SVCR) & SVCR_SM_MASK)
> +
> +#define vcpu_za_enabled(vcpu) \
> +	(__vcpu_sys_reg(vcpu, SVCR) & SVCR_ZA_MASK)
> +
>  static inline bool kvm_arch_has_irq_bypass(void)
>  {
>  	return true;
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 1b4cacb6e918..f94fe57adcad 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1948,7 +1948,7 @@ static unsigned int sve_visibility(const struct kvm_vcpu *vcpu,
>  static unsigned int sme_visibility(const struct kvm_vcpu *vcpu,
>  				   const struct sys_reg_desc *rd)
>  {
> -	if (kvm_has_feat(vcpu->kvm, ID_AA64PFR1_EL1, SME, IMP))
> +	if (vcpu_has_sme(vcpu))
>  		return 0;
>  
>  	return REG_HIDDEN;
> 
> -- 
> 2.47.3
> 
>