Re: [PATCH V3 0/2] improve -overcommit cpu-pm=on|off

[Igor Mammedov]

On Mon,  3 Jun 2024 17:02:20 -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.
> 
> Typically #UD takes priority over VM-Exit interception checks and
> KVM doesn't emulate MONITOR/MWAIT. This causes the guest fail to
> boot.
> 
> V2:
> - [PATCH 1]: took Thomas' suggestion for more generic fix
> - [PATCH 2/3]: no changes
> 
> V3:
> - dropped [PATCH 1/3]. Took the simpler approach not to re-order
>   cpu_exec_realizefn() call.
> - changed patch title in [PATCH V3 1/2]
> - don't set CPUID_EXT_MONITOR in kvm_cpu_realizefn() 

on top of above we should make make
  -overcommit cpu-pm=on
to error out if KVM_X86_DISABLE_EXITS_MWAIT is not supported/failed

if we don't do this user gets false assumption that cpu-pm=on
works as expected, and instead of effective CPU usage/IPI delivery
all they get is a storm of mwait exits.

> 
> Zide Chen (2):
>   vl: Allow multiple -overcommit commands
>   target/i386: Advertise MWAIT iff host supports
> 
>  system/vl.c               |  4 ++--
>  target/i386/host-cpu.c    | 12 ------------
>  target/i386/kvm/kvm-cpu.c | 11 +++++++++--
>  3 files changed, 11 insertions(+), 16 deletions(-)
>