KVM: SVM: Document the ASID logic in svm_flush_tlb()

Add a comment in svm_flush_tlb() to document why it flushes only the
current ASID, even when it is invoked when flushing remote TLBs.

Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200320212833.3507-22-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/svm.c

@@ -3157,6 +3157,13 @@ void svm_flush_tlb(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
+	/*
+	 * Flush only the current ASID even if the TLB flush was invoked via
+	 * kvm_flush_remote_tlbs().  Although flushing remote TLBs requires all
+	 * ASIDs to be flushed, KVM uses a single ASID for L1 and L2, and
+	 * unconditionally does a TLB flush on both nested VM-Enter and nested
+	 * VM-Exit (via kvm_mmu_reset_context()).
+	 */
 	if (static_cpu_has(X86_FEATURE_FLUSHBYASID))
 		svm->vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
 	else