KVM: PPC: Use READ_ONCE when dereferencing pte_t pointer

pte can get updated from other CPUs as part of multiple activities
like THP split, huge page collapse, unmap. We need to make sure we
don't reload the pte value again and again for different checks.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/kvm_book3s_64.h

@@ -290,7 +290,10 @@ static inline pte_t kvmppc_read_update_linux_pte(pte_t *ptep, int writing,
 	pte_t old_pte, new_pte = __pte(0);
 
 	while (1) {
-		old_pte = *ptep;
+		/*
+		 * Make sure we don't reload from ptep
+		 */
+		old_pte = READ_ONCE(*ptep);
 		/*
 		 * wait until _PAGE_BUSY is clear then set it atomically
 		 */

arch/powerpc/kvm/e500_mmu_host.c

@@ -469,14 +469,18 @@ static inline int kvmppc_e500_shadow_map(struct kvmppc_vcpu_e500 *vcpu_e500,
 
 	pgdir = vcpu_e500->vcpu.arch.pgdir;
 	ptep = lookup_linux_ptep(pgdir, hva, &tsize_pages);
-	if (pte_present(*ptep))
-		wimg = (*ptep >> PTE_WIMGE_SHIFT) & MAS2_WIMGE_MASK;
-	else {
-		if (printk_ratelimit())
-			pr_err("%s: pte not present: gfn %lx, pfn %lx\n",
-				__func__, (long)gfn, pfn);
-		ret = -EINVAL;
-		goto out;
+	if (ptep) {
+		pte_t pte = READ_ONCE(*ptep);
+
+		if (pte_present(pte))
+			wimg = (pte_val(pte) >> PTE_WIMGE_SHIFT) &
+				MAS2_WIMGE_MASK;
+		else {
+			pr_err_ratelimited("%s: pte not present: gfn %lx,pfn %lx\n",
+					   __func__, (long)gfn, pfn);
+			ret = -EINVAL;
+			goto out;
+		}
 	}
 	kvmppc_e500_ref_setup(ref, gtlbe, pfn, wimg);