Re: [PATCH v22 22/23] KVM: arm64: CCA: Expose SVE VL register before VCPU finalization

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

On 06/10/2026 06:35, Gavin Shan wrote:
> On 10/5/26 7:07 PM, Suzuki K Poulose wrote:
>> From: Jean-Philippe Brucker <jean-philippe@linaro.org>
>>
>> Userspace must configure the SVE vector length before the Realm is 
>> created
>> (as it is part of the parameter for Realm creation), but the Realm VCPUs
>> cannot be finalized until after the Realm Descriptor has been created.
>>
>> KVM_GET_REG_LIST currently rejects the unfinalized VCPUs, which prevents
>> the userspace from discovering and configuring the VLs for  the Realm.
>>
>> Allow KVM_GET_REG_LIST for unfinalized RECs and make the SVE register
>> enumeration handle the unfinalized case explicitly. i.e., only expose
>> KVM_REG_ARM64_SVE_VLS before SVE is finalized.
>>
>> One adverse side effect of this change is that a KVM_GET_REG_LIST call 
>> that
>> only probes for the array size will now succeed even if SVE is not
>> finalized, but that seems harmless since the following KVM_GET_REG_LIST
>> with the full array will fail.
>>
>> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
>> Signed-off-by: Steven Price <steven.price@arm.com>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> ---
>>   arch/arm64/kvm/arm.c   | 15 ++++++++++++++-
>>   arch/arm64/kvm/guest.c | 10 +++++-----
>>   2 files changed, 19 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>> index fe707a0c47308..d99e7818f5894 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -2004,6 +2004,19 @@ static int kvm_arm_vcpu_set_events(struct 
>> kvm_vcpu *vcpu,
>>       return __kvm_arm_vcpu_set_events(vcpu, events);
>>   }
>> +/*
>> + * Realm VCPUs can be finalized only after the Realm descriptor is 
>> created.
>> + * But in order to seal the SVE VL, we need to allow the userspace to 
>> read/write
>> + * to the SVE_VL, before everything is finalized.
>> + * Allow the register list for RECs before the VCPUs are finalized.
>> + */
>> +static bool kvm_arm_vcpu_reg_list_allowed(struct kvm_vcpu *vcpu)
>> +{
>> +    if (kvm_arm_vcpu_is_finalized(vcpu))
>> +        return true;
>> +    return vcpu_is_rec(vcpu);
>> +}
>> +
> 
> Needn't to keep this helper, and the code can be integrated to 
> kvm_arch_vcpu_ioctl(),
> see below.
> 
>>   long kvm_arch_vcpu_ioctl(struct file *filp,
>>                unsigned int ioctl, unsigned long arg)
>>   {
>> @@ -2059,7 +2072,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>>               break;
>>           r = -EPERM;
>> -        if (!kvm_arm_vcpu_is_finalized(vcpu))
>> +        if (!kvm_arm_vcpu_reg_list_allowed(vcpu))
>>               break;
> 
> Needn't to keep the helper kvm_arm_vcpu_reg_list_allowed() after its 
> logic is
> combined to kvm_arch_vcpu_ioctl().
> 
>          /*
>           * Realm vCPUs can be finalized only after the Realm descriptor 
> is created.
>           * We need to allow access KVM_REG_ARM64_SVE_VLS before that so 
> that the
>           * register can be sealed.
>           */
>          if (!(vcpu_is_rec(vcpu) || kvm_arm_vcpu_is_finalized(vcpu))
>              break;


Wanted to keep the ioctl handling section cleaner and easier to read.
Hence the wrapper. The name is intuitive enough and compiler can do
away with inlining.

> 
>>           r = -EFAULT;
>> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
>> index b01d6622b8720..c3ca369882273 100644
>> --- a/arch/arm64/kvm/guest.c
>> +++ b/arch/arm64/kvm/guest.c
>> @@ -598,8 +598,8 @@ static unsigned long num_sve_regs(const struct 
>> kvm_vcpu *vcpu)
>>       if (!vcpu_has_sve(vcpu))
>>           return 0;
>> -    /* Policed by KVM_GET_REG_LIST: */
>> -    WARN_ON(!kvm_arm_vcpu_sve_finalized(vcpu));
>> +    if (!kvm_arm_vcpu_sve_finalized(vcpu))
>> +        return 1; /* KVM_REG_ARM64_SVE_VLS */
> 
> Question: After a (Rec) vCPU is finalized, the returned number of 
> registers won't
> be 1. Is this expected? Otherwise, we need to use vcpu_is_rec() here.
> 
>      /*
>       * KVM_REG_ARM64_SVE_VLS is visible for realm vCPUs no matter if
>       * they have been finalized.
>       */
>      if (vcpu_is_rec(vcpu))
>          return 1;

This is addressed in the next patch.

Cheers
Suzuki