KVM: x86: Optimization: Requst TLB flush in fast_cr3_switch() instead of do it directly

When KVM emulates a nested VMEntry (L1->L2 VMEntry), it switches mmu root
page. If nEPT is used, this will happen from
kvm_init_shadow_ept_mmu()->__kvm_mmu_new_cr3() and otherwise it will
happpen from nested_vmx_load_cr3()->kvm_mmu_new_cr3(). Either case,
__kvm_mmu_new_cr3() will use fast_cr3_switch() in attempt to switch to a
previously cached root page.

In case fast_cr3_switch() finds a matching cached root page, it will
set it in mmu->root_hpa and request KVM_REQ_LOAD_CR3 such that on
next entry to guest, KVM will set root HPA in appropriate hardware
fields (e.g. vmcs->eptp). In addition, fast_cr3_switch() calls
kvm_x86_ops->tlb_flush() in order to flush TLB as MMU root page
was replaced.

This works as mmu->root_hpa, which vmx_flush_tlb() use, was
already replaced in cached_root_available(). However, this may
result in unnecessary INVEPT execution because a KVM_REQ_TLB_FLUSH
may have already been requested. For example, by prepare_vmcs02()
in case L1 don't use VPID.

Therefore, change fast_cr3_switch() to just request TLB flush on
next entry to guest.

Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>