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

[Steven Price]

On 19/03/2026 18:50, Wei-Lin Chang wrote:
> 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?

Yes I agree that would be more consistent.

Thanks,
Steve

> Thanks,
> Wei-Lin Chang
> 
>>  }
>>  
>>  /*
>> -- 
>> 2.43.0
>>