arm64: KVM: add missing dsb before invalidating Stage-2 TLBs

When performing a Stage-2 TLB invalidation, it is necessary to
make sure the write to the page tables is observable by all CPUs.

For this purpose, add dsb instructions to __kvm_tlb_flush_vmid_ipa
and __kvm_flush_vm_context before doing the TLB invalidation itself.
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm64/kvm/hyp.S

@@ -604,6 +604,8 @@ END(__kvm_vcpu_run)
 
 // void __kvm_tlb_flush_vmid_ipa(struct kvm *kvm, phys_addr_t ipa);
 ENTRY(__kvm_tlb_flush_vmid_ipa)
+	dsb	ishst
+
 	kern_hyp_va	x0
 	ldr	x2, [x0, #KVM_VTTBR]
 	msr	vttbr_el2, x2
@@ -625,6 +627,7 @@ ENTRY(__kvm_tlb_flush_vmid_ipa)
 ENDPROC(__kvm_tlb_flush_vmid_ipa)
 
 ENTRY(__kvm_flush_vm_context)
+	dsb	ishst
 	tlbi	alle1is
 	ic	ialluis
 	dsb	sy