i386: Support KVM_CAP_HYPERV_ENFORCE_CPUID

By default, KVM allows the guest to use all currently supported Hyper-V
enlightenments when Hyper-V CPUID interface was exposed, regardless of if
some features were not announced in guest visible CPUIDs. hv-enforce-cpuid
feature alters this behavior and only allows the guest to use exposed
Hyper-V enlightenments. The feature is supported by Linux >= 5.14 and is
not enabled by default in QEMU.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210902093530.345756-5-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

4 files changed, 25 insertions, 3 deletions

diff --git a/docs/hyperv.txt b/docs/hyperv.txt
index 000638a..072709a 100644
--- a/docs/hyperv.txt
+++ b/docs/hyperv.txt
@@ -203,8 +203,11 @@ When the option is set to 'on' QEMU will always enable the feature, regardless
 of host setup. To keep guests secure, this can only be used in conjunction with
 exposing correct vCPU topology and vCPU pinning.
 
-4. Development features
-========================
+4. Supplementary features
+=========================
+
+4.1. hv-passthrough
+===================
 In some cases (e.g. during development) it may make sense to use QEMU in
 'pass-through' mode and give Windows guests all enlightenments currently
 supported by KVM. This pass-through mode is enabled by "hv-passthrough" CPU
@@ -215,8 +218,16 @@ values from KVM to QEMU. "hv-passthrough" overrides all other "hv-*" settings on
 the command line. Also, enabling this flag effectively prevents migration as the
 list of enabled enlightenments may differ between target and destination hosts.
 
+4.2. hv-enforce-cpuid
+=====================
+By default, KVM allows the guest to use all currently supported Hyper-V
+enlightenments when Hyper-V CPUID interface was exposed, regardless of if
+some features were not announced in guest visible CPUIDs. 'hv-enforce-cpuid'
+feature alters this behavior and only allows the guest to use exposed Hyper-V
+enlightenments.
+
 
-4. Useful links
+5. Useful links
 ================
 Hyper-V Top Level Functional specification and other information:
 https://github.com/MicrosoftDocs/Virtualization-Documentation
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 598019d..2a19eba 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -6834,6 +6834,7 @@ static Property x86_cpu_properties[] = {
     DEFINE_PROP_ON_OFF_AUTO("hv-no-nonarch-coresharing", X86CPU,
                             hyperv_no_nonarch_cs, ON_OFF_AUTO_OFF),
     DEFINE_PROP_BOOL("hv-passthrough", X86CPU, hyperv_passthrough, false),
+    DEFINE_PROP_BOOL("hv-enforce-cpuid", X86CPU, hyperv_enforce_cpuid, false),
 
     DEFINE_PROP_BOOL("check", X86CPU, check_cpuid, true),
     DEFINE_PROP_BOOL("enforce", X86CPU, enforce_cpuid, false),
diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index c990150..8a7209b 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -1719,6 +1719,7 @@ struct X86CPU {
     uint32_t hyperv_version_id[4];
     uint32_t hyperv_limits[3];
     uint32_t hyperv_nested[4];
+    bool hyperv_enforce_cpuid;
 
     bool check_cpuid;
     bool enforce_cpuid;
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index d6a70c2..fbe6b7a 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -1531,6 +1531,15 @@ static int hyperv_init_vcpu(X86CPU *cpu)
         cpu->hyperv_nested[0] = evmcs_version;
     }
 
+    if (cpu->hyperv_enforce_cpuid) {
+        ret = kvm_vcpu_enable_cap(cs, KVM_CAP_HYPERV_ENFORCE_CPUID, 0, 1);
+        if (ret < 0) {
+            error_report("failed to enable KVM_CAP_HYPERV_ENFORCE_CPUID: %s",
+                         strerror(-ret));
+            return ret;
+        }
+    }
+
     return 0;
 }