KVM: VMX: Remove unnecessary INVEPT[GLOBAL] from hardware enable path

Remove the completely pointess global INVEPT, i.e. EPT TLB flush, from
KVM's VMX enablement path.  KVM always does a targeted TLB flush when
using a "new" EPT root, in quotes because "new" simply means a root that
isn't currently being used by the vCPU.

KVM also _deliberately_ runs with stale TLB entries for defunct roots,
i.e. doesn't do a TLB flush when vCPUs stop using roots, precisely because
KVM does the flush on first use.  As called out by the comment in
kvm_mmu_load(), the reason KVM flushes on first use is because KVM can't
guarantee the correctness of past hypervisors.

Jumping back to the global INVEPT, when the painfully terse commit
1439442c ("KVM: VMX: Enable EPT feature for KVM") was added, the
effective TLB flush being performed was:

  static void vmx_flush_tlb(struct kvm_vcpu *vcpu)
  {
          vpid_sync_vcpu_all(to_vmx(vcpu));
  }

I.e. KVM was not flushing EPT TLB entries when allocating a "new" root,
which very strongly suggests that the global INVEPT during hardware
enabling was a misguided hack that addressed the most obvious symptom,
but failed to fix the underlying bug.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20240608001003.3296640-1-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>