LoongArch: KVM: Add memory barrier before update pmd entry

When updating pmd entry such as allocating new pmd page or splitting
huge page into normal page, it is necessary to firstly update all pte
entries, and then update pmd entry.

It is weak order with LoongArch system, there will be problem if other
VCPUs see pmd update firstly while ptes are not updated. Here smp_wmb()
is added to assure this.
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/kvm/mmu.c

@@ -163,6 +163,7 @@ static kvm_pte_t *kvm_populate_gpa(struct kvm *kvm,
 
 			child = kvm_mmu_memory_cache_alloc(cache);
 			_kvm_pte_init(child, ctx.invalid_ptes[ctx.level - 1]);
+			smp_wmb(); /* Make pte visible before pmd */
 			kvm_set_pte(entry, __pa(child));
 		} else if (kvm_pte_huge(*entry)) {
 			return entry;
@@ -746,6 +747,7 @@ static kvm_pte_t *kvm_split_huge(struct kvm_vcpu *vcpu, kvm_pte_t *ptep, gfn_t g
 		val += PAGE_SIZE;
 	}
 
+	smp_wmb(); /* Make pte visible before pmd */
 	/* The later kvm_flush_tlb_gpa() will flush hugepage tlb */
 	kvm_set_pte(ptep, __pa(child));