Re: [PATCH V2 0/3] improve -overcommit cpu-pm=on|off

["Chen, Zide" <zide.chen@intel.com>]

On 5/28/2024 2:23 AM, Igor Mammedov wrote:
> On Fri, 24 May 2024 13:00:14 -0700
> Zide Chen <zide.chen@intel.com> wrote:
> 
>> Currently, if running "-overcommit cpu-pm=on" on hosts that don't
>> have MWAIT support, the MWAIT/MONITOR feature is advertised to the
>> guest and executing MWAIT/MONITOR on the guest triggers #UD.
> 
> this is missing proper description how do you trigger issue
> with reproducer and detailed description why guest sees MWAIT
> when it's not supported by host.

If "overcommit cpu-pm=on" and "-cpu hpst" are present, as shown in the
following, CPUID_EXT_MONITOR is set after x86_cpu_filter_features(), so
that it doesn't have a chance to check MWAIT against host features and
will be advertised to the guest regardless of whether it's supported by
the host or not.

x86_cpu_realizefn()
  x86_cpu_filter_features()
  cpu_exec_realizefn()
    kvm_cpu_realizefn
      host_cpu_realizefn
        host_cpu_enable_cpu_pm
          env->features[FEAT_1_ECX] |= CPUID_EXT_MONITOR;


If it's not supported by the host, executing MONITOR or MWAIT
instructions from the guest triggers #UD, no matter MWAIT_EXITING
control is set or not.