[PATCH v2 3/8] i386: Support KVM_CAP_ENFORCE_PV_FEATURE_CPUID

[Vitaly Kuznetsov <vkuznets@redhat.com>]

By default, KVM allows the guest to use all currently supported PV features
even when they were not announced in guest visible CPUIDs. Introduce a new
"kvm-pv-enforce-cpuid" flag to limit the supported feature set to the
exposed features. The feature is supported by Linux >= 5.10 and is not
enabled by default in QEMU.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 docs/kvm-pv.txt       | 13 ++++++++++++-
 target/i386/cpu.c     |  2 ++
 target/i386/cpu.h     |  3 +++
 target/i386/kvm/kvm.c | 10 ++++++++++
 4 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/docs/kvm-pv.txt b/docs/kvm-pv.txt
index 84ad7fa60f8d..d1aac533feea 100644
--- a/docs/kvm-pv.txt
+++ b/docs/kvm-pv.txt
@@ -87,6 +87,17 @@ the number of supported vCPUs for a given configuration lower).
 Tells the guest that guest visible TSC value can be fully trusted for kvmclock
 computations and no warps are expected.
 
-4. Useful links
+4. Supplementary features
+=========================
+
+4.1. kvm-pv-enforce-cpuid
+=========================
+By default, KVM allows the guest to use all currently supported PV features even
+when they were not announced in guest visible CPUIDs. 'kvm-pv-enforce-cpuid'
+feature alters this behavior and limits the supported feature set to the
+exposed features only.
+
+
+5. Useful links
 ================
 Please refer to Documentation/virt/kvm in Linux for additional detail.
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 97e250e8760d..a70038f172d9 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -6691,6 +6691,8 @@ static Property x86_cpu_properties[] = {
     DEFINE_PROP_BOOL("l3-cache", X86CPU, enable_l3_cache, true),
     DEFINE_PROP_BOOL("kvm-no-smi-migration", X86CPU, kvm_no_smi_migration,
                      false),
+    DEFINE_PROP_BOOL("kvm-pv-enforce-cpuid", X86CPU, kvm_pv_enforce_cpuid,
+                     false),
     DEFINE_PROP_BOOL("vmware-cpuid-freq", X86CPU, vmware_cpuid_freq, true),
     DEFINE_PROP_BOOL("tcg-cpuid", X86CPU, expose_tcg, true),
     DEFINE_PROP_BOOL("x-migrate-smi-count", X86CPU, migrate_smi_count,
diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 6c50d3ab4f1d..20273a8069dd 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -1782,6 +1782,9 @@ struct X86CPU {
     /* Stop SMI delivery for migration compatibility with old machines */
     bool kvm_no_smi_migration;
 
+    /* Forcefully disable KVM PV features not exposed in guest CPUIDs */
+    bool kvm_pv_enforce_cpuid;
+
     /* Number of physical address bits supported */
     uint32_t phys_bits;
 
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 500d2e0e686f..49f97f345069 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -1629,6 +1629,16 @@ int kvm_arch_init_vcpu(CPUState *cs)
 
     cpu_x86_cpuid(env, 0, 0, &limit, &unused, &unused, &unused);
 
+    if (cpu->kvm_pv_enforce_cpuid) {
+        r = kvm_vcpu_enable_cap(cs, KVM_CAP_ENFORCE_PV_FEATURE_CPUID, 0, 1);
+        if (r < 0) {
+            fprintf(stderr,
+                    "failed to enable KVM_CAP_ENFORCE_PV_FEATURE_CPUID: %s",
+                    strerror(-r));
+            abort();
+        }
+    }
+
     for (i = 0; i <= limit; i++) {
         if (cpuid_i == KVM_MAX_CPUID_ENTRIES) {
             fprintf(stderr, "unsupported level value: 0x%x\n", limit);
-- 
2.31.1