Re: [RFC PATCH v7 18/18] arm-qmp-cmds: introspection for ID register props

> On 26 Jul 2026, at 8:59 PM, Eric Auger <[email protected]> wrote:
> 
> !-------------------------------------------------------------------|
>  CAUTION: External Email
> 
> |-------------------------------------------------------------------!
> 
> From: Cornelia Huck <[email protected]>
> 
> Implement the capability to query available ID register values by
> adding SYSREG_* options and values to the cpu model expansion for the
> host model, if available.
> 
> Excerpt:
> (QEMU) query-cpu-model-expansion type=full model={"name":"host"}
> {"return": {"model": {"name": "host", "props": {"SYSREG_ID_AA64PFR0_EL1_EL3": 1,
> "SYSREG_ID_AA64ISAR2_EL1_CLRBHB": 0, "SYSREG_CTR_EL0_L1Ip": 3,
> "SYSREG_MIDR_EL1_PartNum": 3407, "SYSREG_CTR_EL0_DminLine": 4,
> "SYSREG_ID_AA64MMFR0_EL1_PARange": 5, "SYSREG_ID_AA64MMFR1_EL1_ECBHB": 0
> ../..
> 
> This allows the upper stack to detect available writable ID regs and
> the "host passthrough model" values.
> 
> It also allows to test some ID reg field values:
> (QEMU) query-cpu-model-expansion type=full model={"name":"host","props":{"SYSREG_ID_AA64ISAR0_EL1_DP":0x13}}
> {"error": {"class": "GenericError", "desc": "idreg SYSREG_ID_AA64ISAR0_EL1_DP set value (0x13) exceeds length of field (4)!"}}
> 
> (QEMU) query-cpu-model-expansion type=full model={"name":"host","props":{"SYSREG_ID_AA64ISAR0_EL1_DP":0x2}}
> {"error": {"class": "GenericError", "desc": "idreg SYSREG_ID_AA64ISAR0_EL1_DP set value (0x2) does not match any arch valid enum value!”}}
My reasoning here is: User never asked to run the CPU model/configuration. The QMP command is supposed to
return the final prop values which QEMU will set/request from KVM.

Like on x86, the query-cpu-model-expansion type=full model={“host”: “props”: {“amx-tile": true}} does not return
with error even if host does not have AMX.

But, maybe we can append CpuModelInfo or CpuModelExpansionInfo with optional ‘usable’ boolean. This even
fills an existing gap, as, query-cpu-definitions cannot take property values.

Warm Regards,
Khushit
> The following checks are performed:
> - the value does not exceed the field size
> - if the field is associated to enum values, the set value must be one
>  of those enum values
> - the value is applied against a scratch vcpu making sure the setting is
>  not rejected for this host
> 
> Signed-off-by: Eric Auger <[email protected]>
> Signed-off-by: Cornelia Huck <[email protected]>
> 
> ---
> 
> v5 -> v6:
> - add the write capability
> ---
> target/arm/arm-qmp-cmds.c | 98 +++++++++++++++++++++++++++++++++++++++
> 1 file changed, 98 insertions(+)
> 
> diff --git a/target/arm/arm-qmp-cmds.c b/target/arm/arm-qmp-cmds.c
> index 83ec95c290..41fefadcea 100644
> --- a/target/arm/arm-qmp-cmds.c
> +++ b/target/arm/arm-qmp-cmds.c
> @@ -21,6 +21,7 @@
>  */
> 
> #include "qemu/osdep.h"
> +#include "qemu/error-report.h"
> #include "qemu/target-info.h"
> #include "hw/core/boards.h"
> #include "kvm_arm.h"
> @@ -30,7 +31,10 @@
> #include "qapi/qapi-commands-machine.h"
> #include "qapi/qapi-commands-misc-arm.h"
> #include "qobject/qdict.h"
> +#include "qobject/qnum.h"
> #include "qom/qom-qobject.h"
> +#include <linux/kvm.h>
> +#include "system/kvm.h"
> #include "cpu.h"
> 
> static GICCapability *gic_cap_new(int version)
> @@ -84,11 +88,14 @@ CpuModelExpansionInfo *qmp_query_cpu_model_expansion(CpuModelExpansionType type,
>                                                      Error **errp)
> {
>     CpuModelExpansionInfo *expansion_info;
> +    ObjectPropertyIterator iter;
>     const QDict *qdict_in;
> +    ObjectProperty *idregprop;
>     QDict *qdict_out;
>     ObjectClass *oc;
>     Object *obj;
>     const char *name;
> +    int fdarray[3];
>     int i;
> 
>     if (type != CPU_MODEL_EXPANSION_TYPE_FULL) {
> @@ -133,6 +140,38 @@ CpuModelExpansionInfo *qmp_query_cpu_model_expansion(CpuModelExpansionType type,
> 
>     obj = object_new(object_class_get_name(oc));
> 
> +    if (kvm_enabled()) {
> +        bool pmuv3_supported = kvm_check_extension(kvm_state, KVM_CAP_ARM_PMU_V3);
> +        bool sve_supported = kvm_check_extension(kvm_state, KVM_CAP_ARM_SVE);
> +        struct kvm_vcpu_init init = { .target = -1, };
> +        bool el2_supported = kvm_arm_el2_supported();
> +        bool pauth_supported;
> +        int ret;
> +
> +        pauth_supported = kvm_check_extension(kvm_state, KVM_CAP_ARM_PTRAUTH_ADDRESS) &&
> +                          kvm_check_extension(kvm_state, KVM_CAP_ARM_PTRAUTH_GENERIC);
> +
> +        if (sve_supported) {
> +            init.features[0] |= 1 << KVM_ARM_VCPU_SVE;
> +        }
> +        if (el2_supported) {
> +            init.features[0] |= 1 << KVM_ARM_VCPU_HAS_EL2;
> +        }
> +        if (pauth_supported) {
> +            init.features[0] |= (1 << KVM_ARM_VCPU_PTRAUTH_ADDRESS |
> +                             1 << KVM_ARM_VCPU_PTRAUTH_GENERIC);
> +        }
> +        if (pmuv3_supported) {
> +            init.features[0] |= 1 << KVM_ARM_VCPU_PMU_V3;
> +        }
> +
> +        ret = kvm_arm_create_scratch_host_vcpu(fdarray, &init);
> +        if (!ret) {
> +            error_setg(errp, "failing creating a scratch vcpu");
> +            return NULL;
> +        }
> +    }
> +
>     if (model->props) {
>         Visitor *visitor;
>         Error *err = NULL;
> @@ -145,6 +184,49 @@ CpuModelExpansionInfo *qmp_query_cpu_model_expansion(CpuModelExpansionType type,
>         }
> 
>         qdict_in = qobject_to(QDict, model->props);
> +
> +        for (const QDictEntry *entry = qdict_first(qdict_in);
> +                 entry != NULL; entry = qdict_next(qdict_in, entry)) {
> +            const char *key = qdict_entry_key(entry);
> +            QObject *val_obj = qdict_entry_value(entry);
> +            ObjectProperty *prop;
> +            Visitor *v;
> +            bool success;
> +            uint64_t val;
> +
> +            prop = object_property_find(obj, key);
> +            if (!prop) {
> +                error_setg(errp, "%s does not exist!", key);
> +                return NULL;
> +            }
> +
> +            if (!g_str_has_prefix(key, "SYSREG_")) {
> +                continue;
> +            }
> +
> +            /* consume the prop to avoid unexpected parameter */
> +            if (!visit_type_uint64(visitor, key, &val, errp)) {
> +                return NULL;
> +            }
> +
> +            v = qobject_input_visitor_new(val_obj);
> +
> +            if (!object_property_set(obj, key, v, &err)) {
> +                error_propagate(errp, err);
> +                visit_free(v);
> +                return NULL;
> +            }
> +
> +            success = kvm_idreg_write_scratch_vcpu(fdarray[2], v, key,
> +                                                   prop->opaque, &err);
> +            if (!success) {
> +                error_propagate(errp, err);
> +                visit_free(v);
> +                return NULL;
> +            }
> +            visit_free(v);
> +        }
> +
>         i = 0;
>         while ((name = cpu_model_advertised_features[i++]) != NULL) {
>             if (qdict_get(qdict_in, name)) {
> @@ -160,6 +242,10 @@ CpuModelExpansionInfo *qmp_query_cpu_model_expansion(CpuModelExpansionType type,
>         if (!err) {
>             arm_cpu_finalize_features(ARM_CPU(obj), &err);
>         }
> +
> +        if (kvm_enabled()) {
> +            kvm_arm_destroy_scratch_host_vcpu(fdarray);
> +        }
>         visit_end_struct(visitor, NULL);
>         visit_free(visitor);
>         if (err) {
> @@ -190,6 +276,18 @@ CpuModelExpansionInfo *qmp_query_cpu_model_expansion(CpuModelExpansionType type,
>         }
>     }
> 
> +    object_property_iter_init(&iter, obj);
> +
> +    while ((idregprop = object_property_iter_next(&iter))) {
> +        QObject *value;
> +
> +        if (!g_str_has_prefix(idregprop->name, "SYSREG_")) {
> +            continue;
> +        }
> +        value = object_property_get_qobject(obj, idregprop->name, &error_abort);
> +        qdict_put_obj(qdict_out, idregprop->name, value);
> +    }
> +
>     if (!qdict_size(qdict_out)) {
>         qobject_unref(qdict_out);
>     } else {
> -- 
> 2.53.0
>