RE: [PATCH] KVM: VMX: Disable Intel PT before VM-entry

Kang, Luwei
2020-04-17T01:50:09Z

> >> Ah, right. What about enhancing intel_pt_handle_vmx() and 'struct
> >> pt' to replace vmx_on with a field that incorporates the KVM mode?
> >
> > Some history is the host perf didn't fully agree with introducing
> > HOST_GUEST mode for PT in KVM.
>
> I don't think this is accurate. IIRC the maintainers wanted packets in the host-
> side trace to signal where the trace was interrupted. In the end we solved the
> issue by 1) dropping host-only mode since it can be achieved in userspace 2)
> making host-guest an opt in feature.
>
> I think it would make sense to rename vmx_on into vmx_state and make it an
>
> enum pt_vmx_state {
> PT_VMX_OFF,
> PT_VMX_ON_DISABLED,
> PT_VMX_ON_SYSTEM,
> PT_VMX_ON_HOST_GUEST
> };
>
> KVM would pass the enum to intel_pt_handle_vmx (one of PT_VMX_OFF,
> PT_VMX_ON_SYSTEM, PT_VMX_ON_HOST_GUEST). Inside
> intel_pt_handle_vmx you can do
>
> if (pt_pmu.vmx) {
> WRITE_ONCE(pt->vmx_state, state);
> return;
> }
>
> local_irq_save(flags);
> WRITE_ONCE(pt->vmx_state,
> state == PT_VMX_OFF ? PT_VMX_OFF :
> PT_VMX_ON_DISABLED);
> ...
>
> and in pt_config_start:
>
> ...
> vmx = READ_ONCE(pt->vmx_start);
> if (vmx == PT_VMX_ON_DISABLED)
> perf_aux_output_flag(&pt->handle, PERF_AUX_FLAG_PARTIAL);
> else if (vmx == PT_VMX_ON_SYSTEM ||
> !(current->flags & PF_VCPU))
> wrmsrl(MSR_IA32_RTIT_CTL, ctl);
> ...

I will try this. Thanks.

Luwei Kang

>
> Thanks,
>
> Paolo