Provide a kvm specific vcpu property to override the default
(as of kernel v6.13 that would be PSCI v1.3) PSCI version emulated
by kvm. Current valid values are: 0.1, 0.2, 1.0, 1.1, 1.2, and 1.3

Note: in order to support PSCI v0.1 we need to drop vcpu
initialization with KVM_CAP_ARM_PSCI_0_2 in that case.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Sebastian Ott <sebott@redhat.com>
---
 docs/system/arm/cpu-features.rst | 11 ++++++
 target/arm/cpu.h                 |  6 +++
 target/arm/kvm.c                 | 65 +++++++++++++++++++++++++++++++-
 3 files changed, 81 insertions(+), 1 deletion(-)

diff --git a/docs/system/arm/cpu-features.rst b/docs/system/arm/cpu-features.rst
index 37d5dfd15b..50b106f2eb 100644
--- a/docs/system/arm/cpu-features.rst
+++ b/docs/system/arm/cpu-features.rst
@@ -204,6 +204,17 @@ the list of KVM VCPU features and their descriptions.
   the guest scheduler behavior and/or be exposed to the guest
   userspace.
 
+``kvm-psci-version``
+  Set the Power State Coordination Interface (PSCI) firmware ABI version
+  that KVM provides to the guest. By default KVM will use the newest
+  version that it knows about (which is PSCI v1.3 in Linux v6.13).
+
+  You only need to set this if you want to be able to migrate this
+  VM to a host machine running an older kernel that does not
+  recognize the PSCI version that this host's kernel defaults to.
+
+  Current valid values are: 0.1, 0.2, 1.0, 1.1, 1.2, and 1.3.
+
 TCG VCPU Features
 =================
 
diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index e146f7e6c4..d3cfd444e8 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -1036,6 +1036,12 @@ struct ArchCPU {
     bool kvm_vtime_dirty;
     uint64_t kvm_vtime;
 
+    /*
+     * Intermediate value used during property parsing.
+     * Once finalized, the value should be read from psci_version.
+     */
+    uint32_t kvm_prop_psci_version;
+
     /* KVM steal time */
     OnOffAuto kvm_steal_time;
 
diff --git a/target/arm/kvm.c b/target/arm/kvm.c
index ded582e0da..6d13cb4f3c 100644
--- a/target/arm/kvm.c
+++ b/target/arm/kvm.c
@@ -485,6 +485,46 @@ static void kvm_steal_time_set(Object *obj, bool value, Error **errp)
     ARM_CPU(obj)->kvm_steal_time = value ? ON_OFF_AUTO_ON : ON_OFF_AUTO_OFF;
 }
 
+typedef struct PSCIVersion {
+    uint32_t number;
+    const char *str;
+} PSCIVersion;
+
+static const PSCIVersion psci_versions[] = {
+    { QEMU_PSCI_VERSION_0_1, "0.1" },
+    { QEMU_PSCI_VERSION_0_2, "0.2" },
+    { QEMU_PSCI_VERSION_1_0, "1.0" },
+    { QEMU_PSCI_VERSION_1_1, "1.1" },
+    { QEMU_PSCI_VERSION_1_2, "1.2" },
+    { QEMU_PSCI_VERSION_1_3, "1.3" },
+};
+
+static char *kvm_get_psci_version(Object *obj, Error **errp)
+{
+    ARMCPU *cpu = ARM_CPU(obj);
+
+    for (int i = 0; i < ARRAY_SIZE(psci_versions); i++) {
+        if (psci_versions[i].number == cpu->psci_version)
+            return g_strdup(psci_versions[i].str);
+    }
+
+    return g_strdup_printf("Unknown PSCI-version: %x", cpu->psci_version);
+}
+
+static void kvm_set_psci_version(Object *obj, const char *value, Error **errp)
+{
+    ARMCPU *cpu = ARM_CPU(obj);
+
+    for (int i = 0; i < ARRAY_SIZE(psci_versions); i++) {
+        if (!strcmp(value, psci_versions[i].str)) {
+            cpu->kvm_prop_psci_version = psci_versions[i].number;
+            return;
+        }
+    }
+
+    error_setg(errp, "Invalid PSCI version.");
+}
+
 /* KVM VCPU properties should be prefixed with "kvm-". */
 void kvm_arm_add_vcpu_properties(ARMCPU *cpu)
 {
@@ -506,6 +546,12 @@ void kvm_arm_add_vcpu_properties(ARMCPU *cpu)
                              kvm_steal_time_set);
     object_property_set_description(obj, "kvm-steal-time",
                                     "Set off to disable KVM steal time.");
+
+    object_property_add_str(obj, "kvm-psci-version", kvm_get_psci_version,
+                            kvm_set_psci_version);
+    object_property_set_description(obj, "kvm-psci-version",
+                                    "Set PSCI version. "
+                                    "Valid values are 0.1, 0.2, 1.0, 1.1, 1.2, 1.3");
 }
 
 bool kvm_arm_pmu_supported(void)
@@ -1976,7 +2022,12 @@ int kvm_arch_init_vcpu(CPUState *cs)
     if (cs->start_powered_off) {
         cpu->kvm_init_features[0] |= 1 << KVM_ARM_VCPU_POWER_OFF;
     }
-    if (kvm_check_extension(cs->kvm_state, KVM_CAP_ARM_PSCI_0_2)) {
+    if (cpu->kvm_prop_psci_version != QEMU_PSCI_VERSION_0_1 &&
+        kvm_check_extension(cs->kvm_state, KVM_CAP_ARM_PSCI_0_2)) {
+        /*
+         * Versions >= v0.2 are backward compatible with v0.2
+         * omit the feature flag for v0.1 .
+         */
         cpu->psci_version = QEMU_PSCI_VERSION_0_2;
         cpu->kvm_init_features[0] |= 1 << KVM_ARM_VCPU_PSCI_0_2;
     }
@@ -2015,6 +2066,18 @@ int kvm_arch_init_vcpu(CPUState *cs)
         }
     }
 
+    if (cpu->kvm_prop_psci_version) {
+        psciver = cpu->kvm_prop_psci_version;
+        ret = kvm_set_one_reg(cs, KVM_REG_ARM_PSCI_VERSION, &psciver);
+        if (ret) {
+            error_report("KVM in this kernel does not support PSCI version %d.%d",
+                         (int) PSCI_VERSION_MAJOR(psciver),
+                         (int) PSCI_VERSION_MINOR(psciver));
+            error_printf("Consider setting the kvm-psci-version property on the "
+                         "migration source.\n");
+            return ret;
+        }
+    }
     /*
      * KVM reports the exact PSCI version it is implementing via a
      * special sysreg. If it is present, use its contents to determine
-- 
2.52.0