target/i386: Advertise MWAIT iff host supports

host_cpu_realizefn() sets CPUID_EXT_MONITOR without consulting host/KVM
capabilities. This may cause problems:

- If MWAIT/MONITOR is not available on the host, advertising this
  feature to the guest and executing MWAIT/MONITOR from the guest
  triggers #UD and the guest doesn't boot.  This is because typically
  #UD takes priority over VM-Exit interception checks and KVM doesn't
  emulate MONITOR/MWAIT on #UD.

- If KVM doesn't support KVM_X86_DISABLE_EXITS_MWAIT, MWAIT/MONITOR
  from the guest are intercepted by KVM, which is not what cpu-pm=on
  intends to do.

In these cases, MWAIT/MONITOR should not be exposed to the guest.

The logic in kvm_arch_get_supported_cpuid() to handle CPUID_EXT_MONITOR
is correct and sufficient, and we can't set CPUID_EXT_MONITOR after
x86_cpu_filter_features().

This was not an issue before commit 662175b91ff ("i386: reorder call to
cpu_exec_realizefn") because the feature added in the accel-specific
realizefn could be checked against host availability and filtered out.

Additionally, it seems not a good idea to handle guest CPUID leaves in
host_cpu_realizefn(), and this patch merges host_cpu_enable_cpu_pm()
into kvm_cpu_realizefn().

Fixes: f5cc5a5c1686 ("i386: split cpu accelerators from cpu.c, using AccelCPUClass")
Fixes: 662175b91ff2 ("i386: reorder call to cpu_exec_realizefn")
Signed-off-by: Zide Chen <zide.chen@intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

target/i386/host-cpu.c

@@ -42,15 +42,6 @@ static uint32_t host_cpu_phys_bits(void)
     return host_phys_bits;
 }
 
-static void host_cpu_enable_cpu_pm(X86CPU *cpu)
-{
-    CPUX86State *env = &cpu->env;
-
-    host_cpuid(5, 0, &cpu->mwait.eax, &cpu->mwait.ebx,
-               &cpu->mwait.ecx, &cpu->mwait.edx);
-    env->features[FEAT_1_ECX] |= CPUID_EXT_MONITOR;
-}
-
 static uint32_t host_cpu_adjust_phys_bits(X86CPU *cpu)
 {
     uint32_t host_phys_bits = host_cpu_phys_bits();
@@ -83,9 +74,6 @@ bool host_cpu_realizefn(CPUState *cs, Error **errp)
     X86CPU *cpu = X86_CPU(cs);
     CPUX86State *env = &cpu->env;
 
-    if (cpu->max_features && enable_cpu_pm) {
-        host_cpu_enable_cpu_pm(cpu);
-    }
     if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
         uint32_t phys_bits = host_cpu_adjust_phys_bits(cpu);
 

target/i386/kvm/kvm-cpu.c

@@ -64,8 +64,15 @@ static bool kvm_cpu_realizefn(CPUState *cs, Error **errp)
      *   cpu_common_realizefn() (via xcc->parent_realize)
      */
     if (cpu->max_features) {
-        if (enable_cpu_pm && kvm_has_waitpkg()) {
-            env->features[FEAT_7_0_ECX] |= CPUID_7_0_ECX_WAITPKG;
+        if (enable_cpu_pm) {
+            if (kvm_has_waitpkg()) {
+                env->features[FEAT_7_0_ECX] |= CPUID_7_0_ECX_WAITPKG;
+            }
+
+            if (env->features[FEAT_1_ECX] & CPUID_EXT_MONITOR) {
+                host_cpuid(5, 0, &cpu->mwait.eax, &cpu->mwait.ebx,
+                           &cpu->mwait.ecx, &cpu->mwait.edx);
+	    }
         }
         if (cpu->ucode_rev == 0) {
             cpu->ucode_rev =