Re: [PATCH v22 23/23] KVM: arm64: CCA: Control user register access for Realms

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 06/10/2026 07:16, Gavin Shan wrote:
> On 10/6/26 4:01 PM, Suzuki K Poulose wrote:
>> On 06/10/2026 06:47, Gavin Shan wrote:
>>> On 10/5/26 7:07 PM, Suzuki K Poulose wrote:
>>>> From: Jean-Philippe Brucker <jean-philippe@linaro.org>
>>>>
>>>> The RMM restricts the access to the register states that the host can
>>>> read/modify for a given Realm.
>>>>
>>>> e.g., At VCPU creation, can modify GPRS (x0-x30) and PC.
>>>>        While servicing SMCCC calls via RSI_HOST_CALL or servicing PSCI
>>>>        requests.
>>>>        MMIO emulation in the unprotected space.
>>>>
>>>> Additionally we use the sysreg configuration to advertise/configure the
>>>> following Realm parameters, which are required before the Realm 
>>>> Descriptor
>>>> is created:
>>>>   - SVE Vector Length
>>>>   - Number of HW Breakpoints/Watchpoints
>>>>   - PMU Counters.
>>>>
>>>> Thus KVM also additionally allows access to ID_AA64DFR0_EL1 and 
>>>> SVE_VLS for
>>>> the configuration of Realm creation parameters. We don't support 
>>>> PMUs for
>>>> the Realm VMs yet, so PMCR is not exposed.
>>>>
>>>> The RMM makes similar restrictions for reading of the guest's registers
>>>> (this is *confidential* compute after all), however we don't impose the
>>>> restriction here. This allows the VMM to read (stale) values from the
>>>> registers which might be useful to read back the initial values even if
>>>> the RMM doesn't provide the latest version. For migration of a realm 
>>>> VM,
>>>> a new interface will be needed so that the VMM can receive an
>>>> (encrypted) blob of the VM's state.
>>>>
>>>> Reflect the above in KVM_GET_REG_LIST, KVM_SET_ONE_REG calls.
>>
>>>>   static int core_reg_size_from_offset(const struct kvm_vcpu *vcpu, 
>>>> u64 off)
>>>>   {
>>>>       int size;
>>>> @@ -553,6 +572,9 @@ static int copy_core_reg_indices(const struct 
>>>> kvm_vcpu *vcpu,
>>>>           u64 reg = KVM_REG_ARM64 | KVM_REG_ARM_CORE | i;
>>>>           int size = core_reg_size_from_offset(vcpu, i);
>>>> +        if (vcpu_is_rec(vcpu) && !kvm_realm_validate_core_reg(i))
>>>> +            continue;
>>>> +
>>>>           if (size < 0)
>>>>               continue;
>>>> @@ -598,6 +620,9 @@ static unsigned long num_sve_regs(const struct 
>>>> kvm_vcpu *vcpu)
>>>>       if (!vcpu_has_sve(vcpu))
>>>>           return 0;
>>>> +    if (kvm_vm_is_realm(vcpu->kvm))
>>>> +        return 1; /* KVM_REG_ARM64_SVE_VLS */
>>>> +

This could be vcpu_is_rec().
>>>>       if (!kvm_arm_vcpu_sve_finalized(vcpu))
>>>>           return 1; /* KVM_REG_ARM64_SVE_VLS */
>>>>
>>>
>>> Aren't above two checks conflicting to each other?
>>
>> Do they? We allow SVE_VLS only for the Realms and we allow that
>> before the vCPUs are finalized. For normal VMs, depending on
>> whether the vcpus are finalized, we either send 1 or the full list.
>>
> 
> num_sve_regs() can be called for 3 cases: (a) non-finalized RECs; (b) 
> finalized
> RECs; (c) Other finalized vCPUs, correct? "if (kvm_vm_is_realm(vcpu- 
>  >kvm))", which
> would be "if (vcpu_is_rec(vcpu))", covers (a) and (b). We needn't the 
> excessive
> check "if (!kvm_arm_vcpu_sve_finalized(vcpu))". So the check would be 
> something
> as below after this series is applied:
> 
>      /*
>       * KVM_REG_ARM64_SVE_VLS is visible on realm vCPU no matter if it
>       * has been finalized.
>       */
>      if (vcpu_is_rec(vcpu))
>          return 1;
> 
> This check "if (vcpu_is_rec(vcpu))" belongs to PATCH[22]. Hope I make 
> myself
> clear this time :)

Sure, these two patches are closely related and may be even could be
folded in. I split it out to make it easier to review.

22: Allow exposing SVE_VLS for unfinalized VCPU RECs
23: Control register accesses includingthe SVE_VLS, but prevent
everything else

Does it help ?

Cheers
Suzuki


> 
>>>
>>>> @@ -625,6 +650,10 @@ static int copy_sve_reg_indices(const struct 
>>>> kvm_vcpu *vcpu,
>>>>           return -EFAULT;
>>>>       ++num_regs;
>>>> +    /* For Realms only support SVE_VLS */
>>>> +    if (kvm_vm_is_realm(vcpu->kvm))
>>>> +        return num_regs;
>>>> +
>>>>       if (!kvm_arm_vcpu_sve_finalized(vcpu))
>>>>           return num_regs;
>>>
>>> Same question here.
>>
>> As above.
>>
>> Suzuki
> 
> Thanks,
> Gavin
>