KVM: x86/mmu: Add module param to force TLB flush on root reuse

Add a module param, flush_on_reuse, to override skip_tlb_flush and
skip_mmu_sync when performing a so called "fast cr3 switch", i.e. when
reusing a cached root.  The primary motiviation for the control is to
provide a fallback mechanism in the event that TLB flushing and/or MMU
sync bugs are exposed/introduced by upcoming changes to stop
unconditionally flushing on nested VMX transitions.

Suggested-by: Jim Mattson <jmattson@google.com>
Suggested-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200320212833.3507-33-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -78,6 +78,9 @@ module_param_cb(nx_huge_pages_recovery_ratio, &nx_huge_pages_recovery_ratio_ops,
 		&nx_huge_pages_recovery_ratio, 0644);
 __MODULE_PARM_TYPE(nx_huge_pages_recovery_ratio, "uint");
 
+static bool __read_mostly force_flush_and_sync_on_reuse;
+module_param_named(flush_on_reuse, force_flush_and_sync_on_reuse, bool, 0644);
+
 /*
  * When setting this variable to true it enables Two-Dimensional-Paging
  * where the hardware walks 2 page tables:
@@ -4318,9 +4321,9 @@ static void __kvm_mmu_new_cr3(struct kvm_vcpu *vcpu, gpa_t new_cr3,
 	 */
 	kvm_make_request(KVM_REQ_LOAD_MMU_PGD, vcpu);
 
-	if (!skip_mmu_sync)
+	if (!skip_mmu_sync || force_flush_and_sync_on_reuse)
 		kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
-	if (!skip_tlb_flush)
+	if (!skip_tlb_flush || force_flush_and_sync_on_reuse)
 		kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
 
 	/*