[PATCH v7 3/9] i386: hardcode supported eVMCS version to '1'

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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>
---
 docs/hyperv.txt       |  2 +-
 target/i386/kvm/kvm.c | 16 +++++++++++-----
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/docs/hyperv.txt b/docs/hyperv.txt
index a51953daa833..000638a2fd38 100644
--- a/docs/hyperv.txt
+++ b/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
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index c676ee8b38a7..d57eede5dc81 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -1490,13 +1490,19 @@ static int hyperv_init_vcpu(X86CPU *cpu)
         ret = kvm_vcpu_enable_cap(cs, KVM_CAP_HYPERV_ENLIGHTENED_VMCS, 0,
                                   (uintptr_t)&evmcs_version);
 
-        if (ret < 0) {
-            fprintf(stderr, "Hyper-V %s is not supported by kernel\n",
-                    kvm_hyperv_properties[HYPERV_FEAT_EVMCS].desc);
+        /*
+         * 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 || (uint8_t)evmcs_version > 1) {
+            error_report("Hyper-V %s verson 1 is not supported by kernel",
+                         kvm_hyperv_properties[HYPERV_FEAT_EVMCS].desc);
             return ret;
         }
-
-        cpu->hyperv_nested[0] = evmcs_version;
+        cpu->hyperv_nested[0] = (1 << 8) | 1;
     }
 
     return 0;
-- 
2.31.1