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

Paolo Bonzini
2020-04-16T14:36:45Z

On 31/03/20 05:29, Kang, Luwei wrote:
>> 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);
...

Thanks,

Paolo