[PATCH V3 1/3] KVM: PPC: Use READ_ONCE when dereferencing pte_t pointer

[Aneesh Kumar K.V]

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 at linux.vnet.ibm.com>
---
NOTE:
The series depends on the patch
 [PATCH 4/6] powerpc: Fix compile errors with STRICT_MM_TYPECHECKS enabled

 arch/powerpc/include/asm/kvm_book3s_64.h |  5 ++++-
 arch/powerpc/kvm/e500_mmu_host.c         | 20 ++++++++++++--------
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/arch/powerpc/include/asm/kvm_book3s_64.h b/arch/powerpc/include/asm/kvm_book3s_64.h
index cc073a7ac2b7..f06820c67175 100644
--- a/arch/powerpc/include/asm/kvm_book3s_64.h
+++ b/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
 		 */
diff --git a/arch/powerpc/kvm/e500_mmu_host.c b/arch/powerpc/kvm/e500_mmu_host.c
index cc536d4a75ef..5840d546aa03 100644
--- a/arch/powerpc/kvm/e500_mmu_host.c
+++ b/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);
 
-- 
2.1.0