Re: [RFC] KVM: optimize the kvm_vcpu_on_spin

[Paolo Bonzini <pbonzini@redhat.com>]

On 31/07/2017 14:27, David Hildenbrand wrote:
>> I'm not sure whether the operation of get the vcpu's priority-level is
>> expensive on all architectures, so I record it in kvm_sched_out() for
>> minimal the extra cycles cost in kvm_vcpu_on_spin().
>>
> as you only care for x86 right now either way, you can directly optimize
> here for the good (here: x86) case (keeping changes and therefore
> possible bugs minimal).

I agree with Cornelia that this is inconsistent, so you shouldn't update
me->in_kernmode in kvm_vcpu_on_spin.  However, get_cpl requires
vcpu_load on Intel x86, so Mike's patch is necessary (vmx_get_cpl ->
vmx_read_guest_seg_ar -> vmcs_read32).

Alternatively, we can add a new callback kvm_x86_ops->sched_out to x86
KVM, and call vmx_get_cpl from the Intel implementation (vmx_sched_out).
 This will cache the result until the next sched_in, so that
kvm_vcpu_on_spin can use it.

Paolo