Re: [PATCH] KVM: VMX: Stop context switching MSR_IA32_UMWAIT_CONTROL

[Xiaoyao Li]

On 6/23/2020 8:51 AM, Sean Christopherson wrote:
> Remove support for context switching between the guest's and host's
> desired UMWAIT_CONTROL.  Propagating the guest's value to hardware isn't
> required for correct functionality, e.g. KVM intercepts reads and writes
> to the MSR, and the latency effects of the settings controlled by the
> MSR are not architecturally visible.
>
> As a general rule, KVM should not allow the guest to control power
> management settings unless explicitly enabled by userspace, e.g. see
> KVM_CAP_X86_DISABLE_EXITS.  E.g. Intel's SDM explicitly states that C0.2
> can improve the performance of SMT siblings.  A devious guest could
> disable C0.2 so as to improve the performance of their workloads at the
> detriment to workloads running in the host or on other VMs.
>
> Wholesale removal of UMWAIT_CONTROL context switching also fixes a race
> condition where updates from the host may cause KVM to enter the guest
> with the incorrect value.  Because updates are are propagated to all
> CPUs via IPI (SMP function callback), the value in hardware may be
> stale with respect to the cached value and KVM could enter the guest
> with the wrong value in hardware.  As above, the guest can't observe the
> bad value, but it's a weird and confusing wart in the implementation.
>
> Removal also fixes the unnecessary usage of VMX's atomic load/store MSR
> lists.  Using the lists is only necessary for MSRs that are required for
> correct functionality immediately upon VM-Enter/VM-Exit, e.g. EFER on
> old hardware, or for MSRs that need to-the-uop precision, e.g. perf
> related MSRs.  For UMWAIT_CONTROL, the effects are only visible in the
> kernel via TPAUSE/delay(), and KVM doesn't do any form of delay in
> vcpu_vmx_run(). 

> Using the atomic lists is undesirable as they are more
> expensive than direct RDMSR/WRMSR.

Do you mean the extra handling of atomic list facility in kvm? Or just 
mean vm-exit/-entry MSR-load/save in VMX hardware is expensive than 
direct RDMSR/WRMSR instruction?