Re: [PATCH v13 39/48] arm64: RMI: Propagate number of breakpoints and watchpoints to userspace

[Wei-Lin Chang]

On Wed, Mar 18, 2026 at 03:54:03PM +0000, Steven Price wrote:
> From: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> 
> The RMM describes the maximum number of BPs/WPs available to the guest
> in the Feature Register 0. Propagate those numbers into ID_AA64DFR0_EL1,
> which is visible to userspace. A VMM needs this information in order to
> set up realm parameters.
> 
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> Signed-off-by: Steven Price <steven.price@xxxxxxx>
> Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> Reviewed-by: Joey Gouly <joey.gouly@xxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_rmi.h |  2 ++
>  arch/arm64/kvm/rmi.c             | 22 ++++++++++++++++++++++
>  arch/arm64/kvm/sys_regs.c        |  2 +-
>  3 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_rmi.h b/arch/arm64/include/asm/kvm_rmi.h
> index 17bb7e2a2aa0..8fb526764c30 100644
> --- a/arch/arm64/include/asm/kvm_rmi.h
> +++ b/arch/arm64/include/asm/kvm_rmi.h
> @@ -87,6 +87,8 @@ struct realm_rec {
>  void kvm_init_rmi(void);
>  u32 kvm_realm_ipa_limit(void);
>  
> +u64 kvm_realm_reset_id_aa64dfr0_el1(const struct kvm_vcpu *vcpu, u64 val);
> +
>  bool kvm_rmi_supports_sve(void);
>  bool kvm_rmi_supports_pmu(void);
>  
> diff --git a/arch/arm64/kvm/rmi.c b/arch/arm64/kvm/rmi.c
> index 8dc090da6e5f..01519d934d3a 100644
> --- a/arch/arm64/kvm/rmi.c
> +++ b/arch/arm64/kvm/rmi.c
> @@ -212,6 +212,28 @@ u32 kvm_realm_ipa_limit(void)
>  	return u64_get_bits(rmm_feat_reg0, RMI_FEATURE_REGISTER_0_S2SZ);
>  }
>  
> +u64 kvm_realm_reset_id_aa64dfr0_el1(const struct kvm_vcpu *vcpu, u64 val)
> +{
> +	u32 bps = u64_get_bits(rmm_feat_reg0, RMI_FEATURE_REGISTER_0_NUM_BPS);
> +	u32 wps = u64_get_bits(rmm_feat_reg0, RMI_FEATURE_REGISTER_0_NUM_WPS);
> +	u32 ctx_cmps;
> +
> +	if (!kvm_is_realm(vcpu->kvm))
> +		return val;
> +
> +	/* Ensure CTX_CMPs is still valid */
> +	ctx_cmps = FIELD_GET(ID_AA64DFR0_EL1_CTX_CMPs, val);
> +	ctx_cmps = min(bps, ctx_cmps);
> +
> +	val &= ~(ID_AA64DFR0_EL1_BRPs_MASK | ID_AA64DFR0_EL1_WRPs_MASK |
> +		 ID_AA64DFR0_EL1_CTX_CMPs);
> +	val |= FIELD_PREP(ID_AA64DFR0_EL1_BRPs_MASK, bps) |
> +	       FIELD_PREP(ID_AA64DFR0_EL1_WRPs_MASK, wps) |
> +	       FIELD_PREP(ID_AA64DFR0_EL1_CTX_CMPs, ctx_cmps);
> +
> +	return val;
> +}
> +
>  static int get_start_level(struct realm *realm)
>  {
>  	return 4 - stage2_pgtable_levels(realm->ia_bits);
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 46f5e2ab3e2c..83b5c36f43bf 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2043,7 +2043,7 @@ static u64 sanitise_id_aa64dfr0_el1(const struct kvm_vcpu *vcpu, u64 val)
>  	/* Hide BRBE from guests */
>  	val &= ~ID_AA64DFR0_EL1_BRBE_MASK;
>  
> -	return val;
> +	return kvm_realm_reset_id_aa64dfr0_el1(vcpu, val);

Hi,

Nit:
In other places we condition on kvm_is_realm() to separate
realm/non-realm paths but here everyone goes into kvm_realm_*, do you
think it's more consistent to move the kvm_is_realm() check out of this
function?

Thanks,
Wei-Lin Chang

>  }
>  
>  /*
> -- 
> 2.43.0
>