i386: hardcode supported eVMCS version to '1'

Currently, the only eVMCS version, supported by KVM (and described in TLFS)
is '1'. When Enlightened VMCS feature is enabled, QEMU takes the supported
eVMCS version range (from KVM_CAP_HYPERV_ENLIGHTENED_VMCS enablement) and
puts it to guest visible CPUIDs. When (and if) eVMCS ver.2 appears a
problem on migration is expected: it doesn't seem to be possible to migrate
from a host supporting eVMCS ver.2 to a host, which only support eVMCS
ver.1.

Hardcode eVMCS ver.1 as the result of 'hv-evmcs' enablement for now. Newer
eVMCS versions will have to have their own enablement options (e.g.
'hv-evmcs=2').

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20210608120817.1325125-4-vkuznets@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

docs/hyperv.txt

@@ -170,7 +170,7 @@ Recommended: hv-frequencies
 3.16. hv-evmcs
 ===============
 The enlightenment is nested specific, it targets Hyper-V on KVM guests. When
-enabled, it provides Enlightened VMCS feature to the guest. The feature
+enabled, it provides Enlightened VMCS version 1 feature to the guest. The feature
 implements paravirtualized protocol between L0 (KVM) and L1 (Hyper-V)
 hypervisors making L2 exits to the hypervisor faster. The feature is Intel-only.
 Note: some virtualization features (e.g. Posted Interrupts) are disabled when

target/i386/kvm/kvm.c

@@ -1409,6 +1409,21 @@ static int hyperv_fill_cpuids(CPUState *cs,
 static Error *hv_passthrough_mig_blocker;
 static Error *hv_no_nonarch_cs_mig_blocker;
 
+/* Checks that the exposed eVMCS version range is supported by KVM */
+static bool evmcs_version_supported(uint16_t evmcs_version,
+                                    uint16_t supported_evmcs_version)
+{
+    uint8_t min_version = evmcs_version & 0xff;
+    uint8_t max_version = evmcs_version >> 8;
+    uint8_t min_supported_version = supported_evmcs_version & 0xff;
+    uint8_t max_supported_version = supported_evmcs_version >> 8;
+
+    return (min_version >= min_supported_version) &&
+        (max_version <= max_supported_version);
+}
+
+#define DEFAULT_EVMCS_VERSION ((1 << 8) | 1)
+
 static int hyperv_init_vcpu(X86CPU *cpu)
 {
     CPUState *cs = CPU(cpu);
@@ -1488,17 +1503,33 @@ static int hyperv_init_vcpu(X86CPU *cpu)
     }
 
     if (hyperv_feat_enabled(cpu, HYPERV_FEAT_EVMCS)) {
-        uint16_t evmcs_version;
+        uint16_t evmcs_version = DEFAULT_EVMCS_VERSION;
+        uint16_t supported_evmcs_version;
 
         ret = kvm_vcpu_enable_cap(cs, KVM_CAP_HYPERV_ENLIGHTENED_VMCS, 0,
-                                  (uintptr_t)&evmcs_version);
+                                  (uintptr_t)&supported_evmcs_version);
 
+        /*
+         * KVM is required to support EVMCS ver.1. as that's what 'hv-evmcs'
+         * option sets. Note: we hardcode the maximum supported eVMCS version
+         * to '1' as well so 'hv-evmcs' feature is migratable even when (and if)
+         * ver.2 is implemented. A new option (e.g. 'hv-evmcs=2') will then have
+         * to be added.
+         */
         if (ret < 0) {
-            fprintf(stderr, "Hyper-V %s is not supported by kernel\n",
-                    kvm_hyperv_properties[HYPERV_FEAT_EVMCS].desc);
+            error_report("Hyper-V %s is not supported by kernel",
+                         kvm_hyperv_properties[HYPERV_FEAT_EVMCS].desc);
             return ret;
         }
 
+        if (!evmcs_version_supported(evmcs_version, supported_evmcs_version)) {
+            error_report("eVMCS version range [%d..%d] is not supported by "
+                         "kernel (supported: [%d..%d])", evmcs_version & 0xff,
+                         evmcs_version >> 8, supported_evmcs_version & 0xff,
+                         supported_evmcs_version >> 8);
+            return -ENOTSUP;
+        }
+
         cpu->hyperv_nested[0] = evmcs_version;
     }