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

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 28 May 2024 11:16:59 -0700
"Chen, Zide" <zide.chen@intel.com> wrote:

> 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
it's bette to provide full QEMU CLI and host/guest kernels used and what
hardware was used if it's relevant so others can reproduce problem.

> 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.

If I recall right, kvm was able to emulate mwait/monitor.
So question is why it leads to exception instead?