Re: [PATCH v4 08/17] target/arm/kvm: Introduce kvm_get_writable_id_regs

[Eric Auger <eric.auger@redhat.com>]

On 5/7/26 12:07 PM, Shameer Kolothum Thodi wrote:
>
>> -----Original Message-----
>> From: Eric Auger <eric.auger@redhat.com>
>> Sent: 03 May 2026 08:33
>> To: eric.auger.pro@gmail.com; eric.auger@redhat.com; qemu-
>> devel@nongnu.org; qemu-arm@nongnu.org; kvmarm@lists.linux.dev;
>> peter.maydell@linaro.org; richard.henderson@linaro.org;
>> cohuck@redhat.com; sebott@redhat.com; Shameer Kolothum Thodi
>> <skolothumtho@nvidia.com>; philmd@linaro.org
>> Cc: maz@kernel.org; oliver.upton@linux.dev; pbonzini@redhat.com;
>> armbru@redhat.com; berrange@redhat.com; abologna@redhat.com;
>> jdenemar@redhat.com
>> Subject: [PATCH v4 08/17] target/arm/kvm: Introduce
>> kvm_get_writable_id_regs
>>
>> External email: Use caution opening links or attachments
>>
>>
>> From: Cornelia Huck <cohuck@redhat.com>
>>
>> Add an helper to retrieve the writable id reg bitmask. The
>> status of the query is stored in the CPU struct so that an
>> an error, if any, can be reported on vcpu realize().
>>
>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
>> ---
>>  target/arm/cpu.h     | 26 ++++++++++++++++++++++++++
>>  target/arm/kvm.c     | 32 ++++++++++++++++++++++++++++++++
>>  target/arm/kvm_arm.h |  3 +++
>>  3 files changed, 61 insertions(+)
>>
>> diff --git a/target/arm/cpu.h b/target/arm/cpu.h
>> index be14a47c35..2aa22360d2 100644
>> --- a/target/arm/cpu.h
>> +++ b/target/arm/cpu.h
>> @@ -866,6 +866,26 @@ typedef struct {
>>      uint32_t map, init, supported;
>>  } ARMVQMap;
>>
>> +typedef enum ARMIdRegsState {
>> +    WRITABLE_ID_REGS_UNKNOWN,
>> +    WRITABLE_ID_REGS_NOT_DISCOVERABLE,
>> +    WRITABLE_ID_REGS_FAILED,
>> +    WRITABLE_ID_REGS_AVAIL,
>> +} ARMIdRegsState;
>> +
>> +/*
>> + * The following structures are for the purpose of mapping the output of
>> + * KVM_ARM_GET_REG_WRITABLE_MASKS that also may cover id registers
>> we do
>> + * not support in QEMU
>> + * ID registers in op0==3, op1=={0,1,3}, crn=0, crm=={0-7}, op2=={0-7},
>> + * as used by the KVM_ARM_GET_REG_WRITABLE_MASKS ioctl call.
>> + */
>> +#define NR_ID_REG_MASKS (3 * 8 * 8)
> Probably can re-use KVM_ARM_FEATURE_ID_RANGE_SIZE here.
> Looks like used for the same purpose in:
> tools/testing/selftests/kvm/arm64/set_id_regs.c

removed. The writable_map array is now dynamically allocated in
kvm_arch_init_vcpu()

Thanks

Eric
>
> Thanks,
> Shameer
>> +
>> +typedef struct IdRegMap {
>> +    uint64_t regs[NR_ID_REG_MASKS]; /* writable masks for registers */
>> +} IdRegMap;
>> +
>>  /* REG is ID_XXX */
>>  #define FIELD_DP64_IDREG(ISAR, REG, FIELD, VALUE)                       \
>>      ({                                                                  \
>> @@ -1054,6 +1074,12 @@ struct ArchCPU {
>>       */
>>      bool host_cpu_probe_failed;
>>
>> +    /*
>> +     * state of writable id regs query used to report an error, if any,
>> +     * on vcpu model realize
>> +     */
>> +    ARMIdRegsState writable_id_regs_status;
>> +
>>      /* QOM property to indicate we should use the back-compat CNTFRQ
>> default */
>>      bool backcompat_cntfrq;
>>
>> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
>> index d4a68874b8..f06a60804d 100644
>> --- a/target/arm/kvm.c
>> +++ b/target/arm/kvm.c
>> @@ -51,6 +51,7 @@ const KVMCapabilityInfo
>> kvm_arch_required_capabilities[] = {
>>  static bool cap_has_mp_state;
>>  static bool cap_has_inject_serror_esr;
>>  static bool cap_has_inject_ext_dabt;
>> +static int cap_writable_id_regs;
>>
>>  /**
>>   * ARMHostCPUFeatures: information about the host CPU (identified
>> @@ -499,6 +500,37 @@ void
>> kvm_arm_set_cpu_features_from_host(ARMCPU *cpu)
>>      env->features = arm_host_cpu_features.features;
>>  }
>>
>> +int kvm_arm_get_writable_id_regs(ARMCPU *cpu, IdRegMap *idregmap)
>> +{
>> +    struct reg_mask_range range = {
>> +        .range = 0, /* up to now only a single range is supported */
>> +        .addr = (uint64_t)idregmap,
>> +    };
>> +    int ret;
>> +
>> +    if (!kvm_enabled()) {
>> +        cpu->writable_id_regs_status =
>> WRITABLE_ID_REGS_NOT_DISCOVERABLE;
>> +        return -ENOSYS;
>> +    }
>> +
>> +    cap_writable_id_regs =
>> +        kvm_check_extension(kvm_state,
>> KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES);
>> +
>> +    if (!cap_writable_id_regs ||
>> +        !(cap_writable_id_regs & (1 << KVM_ARM_FEATURE_ID_RANGE))) {
>> +        cpu->writable_id_regs_status =
>> WRITABLE_ID_REGS_NOT_DISCOVERABLE;
>> +        return -ENOSYS;
>> +    }
>> +
>> +    ret = kvm_vm_ioctl(kvm_state, KVM_ARM_GET_REG_WRITABLE_MASKS,
>> &range);
>> +    if (ret) {
>> +        cpu->writable_id_regs_status = WRITABLE_ID_REGS_FAILED;
>> +        return ret;
>> +     }
>> +    cpu->writable_id_regs_status = WRITABLE_ID_REGS_AVAIL;
>> +    return ret;
>> +}
>> +
>>  static bool kvm_no_adjvtime_get(Object *obj, Error **errp)
>>  {
>>      return !ARM_CPU(obj)->kvm_adjvtime;
>> diff --git a/target/arm/kvm_arm.h b/target/arm/kvm_arm.h
>> index e7c40fb003..b22a56fc17 100644
>> --- a/target/arm/kvm_arm.h
>> +++ b/target/arm/kvm_arm.h
>> @@ -240,4 +240,7 @@ void arm_gic_cap_kvm_probe(GICCapability *v2,
>> GICCapability *v3);
>>   */
>>  char *kvm_print_register_name(uint64_t regidx);
>>
>> +typedef struct IdRegMap IdRegMap;
>> +int kvm_arm_get_writable_id_regs(ARMCPU *cpu, IdRegMap *idregmap);
>> +
>>  #endif
>> --
>> 2.53.0