KVM: PPC: Book3S PR: Better handling of host-side read-only pages

Currently we request write access to all pages that get mapped into the
guest, even if the guest is only loading from the page.  This reduces
the effectiveness of KSM because it means that we unshare every page we
access.  Also, we always set the changed (C) bit in the guest HPTE if
it allows writing, even for a guest load.

This fixes both these problems.  We pass an 'iswrite' flag to the
mmu.xlate() functions and to kvmppc_mmu_map_page() to indicate whether
the access is a load or a store.  The mmu.xlate() functions now only
set C for stores.  kvmppc_gfn_to_pfn() now calls gfn_to_pfn_prot()
instead of gfn_to_pfn() so that it can indicate whether we need write
access to the page, and get back a 'writable' flag to indicate whether
the page is writable or not.  If that 'writable' flag is clear, we then
make the host HPTE read-only even if the guest HPTE allowed writing.

This means that we can get a protection fault when the guest writes to a
page that it has mapped read-write but which is read-only on the host
side (perhaps due to KSM having merged the page).  Thus we now call
kvmppc_handle_pagefault() for protection faults as well as HPTE not found
faults.  In kvmppc_handle_pagefault(), if the access was allowed by the
guest HPTE and we thus need to install a new host HPTE, we then need to
remove the old host HPTE if there is one.  This is done with a new
function, kvmppc_mmu_unmap_page(), which uses kvmppc_mmu_pte_vflush() to
find and remove the old host HPTE.

Since the memslot-related functions require the KVM SRCU read lock to
be held, this adds srcu_read_lock/unlock pairs around the calls to
kvmppc_handle_pagefault().

Finally, this changes kvmppc_mmu_book3s_32_xlate_pte() to not ignore
guest HPTEs that don't permit access, and to return -EPERM for accesses
that are not permitted by the page protections.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>

9 files changed, 91 insertions, 40 deletions

diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
index 603fba494a0b..a07bd7e7d4a4 100644
--- a/arch/powerpc/include/asm/kvm_book3s.h
+++ b/arch/powerpc/include/asm/kvm_book3s.h
@@ -128,7 +128,9 @@ extern void kvmppc_set_pvr(struct kvm_vcpu *vcpu, u32 pvr);
 extern void kvmppc_mmu_book3s_64_init(struct kvm_vcpu *vcpu);
 extern void kvmppc_mmu_book3s_32_init(struct kvm_vcpu *vcpu);
 extern void kvmppc_mmu_book3s_hv_init(struct kvm_vcpu *vcpu);
-extern int kvmppc_mmu_map_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *pte);
+extern int kvmppc_mmu_map_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *pte,
+			       bool iswrite);
+extern void kvmppc_mmu_unmap_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *pte);
 extern int kvmppc_mmu_map_segment(struct kvm_vcpu *vcpu, ulong eaddr);
 extern void kvmppc_mmu_flush_segment(struct kvm_vcpu *vcpu, ulong eaddr, ulong seg_size);
 extern void kvmppc_mmu_flush_segments(struct kvm_vcpu *vcpu);
@@ -157,7 +159,8 @@ extern void kvmppc_set_bat(struct kvm_vcpu *vcpu, struct kvmppc_bat *bat,
 			   bool upper, u32 val);
 extern void kvmppc_giveup_ext(struct kvm_vcpu *vcpu, ulong msr);
 extern int kvmppc_emulate_paired_single(struct kvm_run *run, struct kvm_vcpu *vcpu);
-extern pfn_t kvmppc_gfn_to_pfn(struct kvm_vcpu *vcpu, gfn_t gfn);
+extern pfn_t kvmppc_gfn_to_pfn(struct kvm_vcpu *vcpu, gfn_t gfn, bool writing,
+			bool *writable);
 extern void kvmppc_add_revmap_chain(struct kvm *kvm, struct revmap_entry *rev,
 			unsigned long *rmap, long pte_index, int realmode);
 extern void kvmppc_invalidate_hpte(struct kvm *kvm, unsigned long *hptep,
diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
index 404dbc81434d..b6881917cd84 100644
--- a/arch/powerpc/include/asm/kvm_host.h
+++ b/arch/powerpc/include/asm/kvm_host.h
@@ -348,7 +348,8 @@ struct kvmppc_mmu {
 	/* book3s */
 	void (*mtsrin)(struct kvm_vcpu *vcpu, u32 srnum, ulong value);
 	u32  (*mfsrin)(struct kvm_vcpu *vcpu, u32 srnum);
-	int  (*xlate)(struct kvm_vcpu *vcpu, gva_t eaddr, struct kvmppc_pte *pte, bool data);
+	int  (*xlate)(struct kvm_vcpu *vcpu, gva_t eaddr,
+		      struct kvmppc_pte *pte, bool data, bool iswrite);
 	void (*reset_msr)(struct kvm_vcpu *vcpu);
 	void (*tlbie)(struct kvm_vcpu *vcpu, ulong addr, bool large);
 	int  (*esid_to_vsid)(struct kvm_vcpu *vcpu, ulong esid, u64 *vsid);
diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
index f97369dc457c..807103ad2628 100644
--- a/arch/powerpc/kvm/book3s.c
+++ b/arch/powerpc/kvm/book3s.c
@@ -286,7 +286,8 @@ int kvmppc_core_prepare_to_enter(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
-pfn_t kvmppc_gfn_to_pfn(struct kvm_vcpu *vcpu, gfn_t gfn)
+pfn_t kvmppc_gfn_to_pfn(struct kvm_vcpu *vcpu, gfn_t gfn, bool writing,
+			bool *writable)
 {
 	ulong mp_pa = vcpu->arch.magic_page_pa;
 
@@ -302,20 +303,22 @@ pfn_t kvmppc_gfn_to_pfn(struct kvm_vcpu *vcpu, gfn_t gfn)
 
 		pfn = (pfn_t)virt_to_phys((void*)shared_page) >> PAGE_SHIFT;
 		get_page(pfn_to_page(pfn));
+		if (writable)
+			*writable = true;
 		return pfn;
 	}
 
-	return gfn_to_pfn(vcpu->kvm, gfn);
+	return gfn_to_pfn_prot(vcpu->kvm, gfn, writing, writable);
 }
 
 static int kvmppc_xlate(struct kvm_vcpu *vcpu, ulong eaddr, bool data,
-			 struct kvmppc_pte *pte)
+			bool iswrite, struct kvmppc_pte *pte)
 {
 	int relocated = (vcpu->arch.shared->msr & (data ? MSR_DR : MSR_IR));
 	int r;
 
 	if (relocated) {
-		r = vcpu->arch.mmu.xlate(vcpu, eaddr, pte, data);
+		r = vcpu->arch.mmu.xlate(vcpu, eaddr, pte, data, iswrite);
 	} else {
 		pte->eaddr = eaddr;
 		pte->raddr = eaddr & KVM_PAM;
@@ -361,7 +364,7 @@ int kvmppc_st(struct kvm_vcpu *vcpu, ulong *eaddr, int size, void *ptr,
 
 	vcpu->stat.st++;
 
-	if (kvmppc_xlate(vcpu, *eaddr, data, &pte))
+	if (kvmppc_xlate(vcpu, *eaddr, data, true, &pte))
 		return -ENOENT;
 
 	*eaddr = pte.raddr;
@@ -383,7 +386,7 @@ int kvmppc_ld(struct kvm_vcpu *vcpu, ulong *eaddr, int size, void *ptr,
 
 	vcpu->stat.ld++;
 
-	if (kvmppc_xlate(vcpu, *eaddr, data, &pte))
+	if (kvmppc_xlate(vcpu, *eaddr, data, false, &pte))
 		goto nopte;
 
 	*eaddr = pte.raddr;
diff --git a/arch/powerpc/kvm/book3s_32_mmu.c b/arch/powerpc/kvm/book3s_32_mmu.c
index b14af6d09347..76a64ce6a5b6 100644
--- a/arch/powerpc/kvm/book3s_32_mmu.c
+++ b/arch/powerpc/kvm/book3s_32_mmu.c
@@ -84,7 +84,8 @@ static inline bool sr_nx(u32 sr_raw)
 }
 
 static int kvmppc_mmu_book3s_32_xlate_bat(struct kvm_vcpu *vcpu, gva_t eaddr,
-					  struct kvmppc_pte *pte, bool data);
+					  struct kvmppc_pte *pte, bool data,
+					  bool iswrite);
 static int kvmppc_mmu_book3s_32_esid_to_vsid(struct kvm_vcpu *vcpu, ulong esid,
 					     u64 *vsid);
 
@@ -99,7 +100,7 @@ static u64 kvmppc_mmu_book3s_32_ea_to_vp(struct kvm_vcpu *vcpu, gva_t eaddr,
 	u64 vsid;
 	struct kvmppc_pte pte;
 
-	if (!kvmppc_mmu_book3s_32_xlate_bat(vcpu, eaddr, &pte, data))
+	if (!kvmppc_mmu_book3s_32_xlate_bat(vcpu, eaddr, &pte, data, false))
 		return pte.vpage;
 
 	kvmppc_mmu_book3s_32_esid_to_vsid(vcpu, eaddr >> SID_SHIFT, &vsid);
@@ -146,7 +147,8 @@ static u32 kvmppc_mmu_book3s_32_get_ptem(u32 sre, gva_t eaddr, bool primary)
 }
 
 static int kvmppc_mmu_book3s_32_xlate_bat(struct kvm_vcpu *vcpu, gva_t eaddr,
-					  struct kvmppc_pte *pte, bool data)
+					  struct kvmppc_pte *pte, bool data,
+					  bool iswrite)
 {
 	struct kvmppc_vcpu_book3s *vcpu_book3s = to_book3s(vcpu);
 	struct kvmppc_bat *bat;
@@ -187,8 +189,7 @@ static int kvmppc_mmu_book3s_32_xlate_bat(struct kvm_vcpu *vcpu, gva_t eaddr,
 				printk(KERN_INFO "BAT is not readable!\n");
 				continue;
 			}
-			if (!pte->may_write) {
-				/* let's treat r/o BATs as not-readable for now */
+			if (iswrite && !pte->may_write) {
 				dprintk_pte("BAT is read-only!\n");
 				continue;
 			}
@@ -202,7 +203,7 @@ static int kvmppc_mmu_book3s_32_xlate_bat(struct kvm_vcpu *vcpu, gva_t eaddr,
 
 static int kvmppc_mmu_book3s_32_xlate_pte(struct kvm_vcpu *vcpu, gva_t eaddr,
 				     struct kvmppc_pte *pte, bool data,
-				     bool primary)
+				     bool iswrite, bool primary)
 {
 	u32 sre;
 	hva_t ptegp;
@@ -258,9 +259,6 @@ static int kvmppc_mmu_book3s_32_xlate_pte(struct kvm_vcpu *vcpu, gva_t eaddr,
 					break;
 			}
 
-			if ( !pte->may_read )
-				continue;
-
 			dprintk_pte("MMU: Found PTE -> %x %x - %x\n",
 				    pteg[i], pteg[i+1], pp);
 			found = 1;
@@ -282,11 +280,12 @@ static int kvmppc_mmu_book3s_32_xlate_pte(struct kvm_vcpu *vcpu, gva_t eaddr,
 			pte_r |= PTEG_FLAG_ACCESSED;
 			put_user(pte_r >> 8, addr + 2);
 		}
-		if (pte->may_write && !(pte_r & PTEG_FLAG_DIRTY)) {
-			/* XXX should only set this for stores */
+		if (iswrite && pte->may_write && !(pte_r & PTEG_FLAG_DIRTY)) {
 			pte_r |= PTEG_FLAG_DIRTY;
 			put_user(pte_r, addr + 3);
 		}
+		if (!pte->may_read || (iswrite && !pte->may_write))
+			return -EPERM;
 		return 0;
 	}
 
@@ -305,7 +304,8 @@ no_page_found:
 }
 
 static int kvmppc_mmu_book3s_32_xlate(struct kvm_vcpu *vcpu, gva_t eaddr,
-				      struct kvmppc_pte *pte, bool data)
+				      struct kvmppc_pte *pte, bool data,
+				      bool iswrite)
 {
 	int r;
 	ulong mp_ea = vcpu->arch.magic_page_ea;
@@ -327,11 +327,13 @@ static int kvmppc_mmu_book3s_32_xlate(struct kvm_vcpu *vcpu, gva_t eaddr,
 		return 0;
 	}
 
-	r = kvmppc_mmu_book3s_32_xlate_bat(vcpu, eaddr, pte, data);
+	r = kvmppc_mmu_book3s_32_xlate_bat(vcpu, eaddr, pte, data, iswrite);
 	if (r < 0)
-	       r = kvmppc_mmu_book3s_32_xlate_pte(vcpu, eaddr, pte, data, true);
+		r = kvmppc_mmu_book3s_32_xlate_pte(vcpu, eaddr, pte,
+						   data, iswrite, true);
 	if (r < 0)
-	       r = kvmppc_mmu_book3s_32_xlate_pte(vcpu, eaddr, pte, data, false);
+		r = kvmppc_mmu_book3s_32_xlate_pte(vcpu, eaddr, pte,
+						   data, iswrite, false);
 
 	return r;
 }
diff --git a/arch/powerpc/kvm/book3s_32_mmu_host.c b/arch/powerpc/kvm/book3s_32_mmu_host.c
index 00e619bf608e..673322329238 100644
--- a/arch/powerpc/kvm/book3s_32_mmu_host.c
+++ b/arch/powerpc/kvm/book3s_32_mmu_host.c
@@ -138,7 +138,8 @@ static u32 *kvmppc_mmu_get_pteg(struct kvm_vcpu *vcpu, u32 vsid, u32 eaddr,
 
 extern char etext[];
 
-int kvmppc_mmu_map_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *orig_pte)
+int kvmppc_mmu_map_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *orig_pte,
+			bool iswrite)
 {
 	pfn_t hpaddr;
 	u64 vpn;
@@ -152,9 +153,11 @@ int kvmppc_mmu_map_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *orig_pte)
 	bool evict = false;
 	struct hpte_cache *pte;
 	int r = 0;
+	bool writable;
 
 	/* Get host physical address for gpa */
-	hpaddr = kvmppc_gfn_to_pfn(vcpu, orig_pte->raddr >> PAGE_SHIFT);
+	hpaddr = kvmppc_gfn_to_pfn(vcpu, orig_pte->raddr >> PAGE_SHIFT,
+				   iswrite, &writable);
 	if (is_error_noslot_pfn(hpaddr)) {
 		printk(KERN_INFO "Couldn't get guest page for gfn %lx!\n",
 				 orig_pte->eaddr);
@@ -204,7 +207,7 @@ next_pteg:
 		(primary ? 0 : PTE_SEC);
 	pteg1 = hpaddr | PTE_M | PTE_R | PTE_C;
 
-	if (orig_pte->may_write) {
+	if (orig_pte->may_write && writable) {
 		pteg1 |= PP_RWRW;
 		mark_page_dirty(vcpu->kvm, orig_pte->raddr >> PAGE_SHIFT);
 	} else {
@@ -259,6 +262,11 @@ out:
 	return r;
 }
 
+void kvmppc_mmu_unmap_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *pte)
+{
+	kvmppc_mmu_pte_vflush(vcpu, pte->vpage, 0xfffffffffULL);
+}
+
 static struct kvmppc_sid_map *create_sid_map(struct kvm_vcpu *vcpu, u64 gvsid)
 {
 	struct kvmppc_sid_map *map;
diff --git a/arch/powerpc/kvm/book3s_64_mmu.c b/arch/powerpc/kvm/book3s_64_mmu.c
index c110145522e6..83da1f868fd5 100644
--- a/arch/powerpc/kvm/book3s_64_mmu.c
+++ b/arch/powerpc/kvm/book3s_64_mmu.c
@@ -206,7 +206,8 @@ static int decode_pagesize(struct kvmppc_slb *slbe, u64 r)
 }
 
 static int kvmppc_mmu_book3s_64_xlate(struct kvm_vcpu *vcpu, gva_t eaddr,
-				struct kvmppc_pte *gpte, bool data)
+				      struct kvmppc_pte *gpte, bool data,
+				      bool iswrite)
 {
 	struct kvmppc_slb *slbe;
 	hva_t ptegp;
@@ -345,8 +346,8 @@ do_second:
 		r |= HPTE_R_R;
 		put_user(r >> 8, addr + 6);
 	}
-	if (data && gpte->may_write && !(r & HPTE_R_C)) {
-		/* Set the dirty flag -- XXX even if not writing */
+	if (iswrite && gpte->may_write && !(r & HPTE_R_C)) {
+		/* Set the dirty flag */
 		/* Use a single byte write */
 		char __user *addr = (char __user *) &pteg[i+1];
 		r |= HPTE_R_C;
@@ -355,7 +356,7 @@ do_second:
 
 	mutex_unlock(&vcpu->kvm->arch.hpt_mutex);
 
-	if (!gpte->may_read)
+	if (!gpte->may_read || (iswrite && !gpte->may_write))
 		return -EPERM;
 	return 0;
 
diff --git a/arch/powerpc/kvm/book3s_64_mmu_host.c b/arch/powerpc/kvm/book3s_64_mmu_host.c
index 6bda504ceda7..cc9fb89b8884 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_host.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_host.c
@@ -78,7 +78,8 @@ static struct kvmppc_sid_map *find_sid_vsid(struct kvm_vcpu *vcpu, u64 gvsid)
 	return NULL;
 }
 
-int kvmppc_mmu_map_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *orig_pte)
+int kvmppc_mmu_map_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *orig_pte,
+			bool iswrite)
 {
 	unsigned long vpn;
 	pfn_t hpaddr;
@@ -91,9 +92,11 @@ int kvmppc_mmu_map_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *orig_pte)
 	struct kvmppc_sid_map *map;
 	int r = 0;
 	int hpsize = MMU_PAGE_4K;
+	bool writable;
 
 	/* Get host physical address for gpa */
-	hpaddr = kvmppc_gfn_to_pfn(vcpu, orig_pte->raddr >> PAGE_SHIFT);
+	hpaddr = kvmppc_gfn_to_pfn(vcpu, orig_pte->raddr >> PAGE_SHIFT,
+				   iswrite, &writable);
 	if (is_error_noslot_pfn(hpaddr)) {
 		printk(KERN_INFO "Couldn't get guest page for gfn %lx!\n", orig_pte->eaddr);
 		r = -EINVAL;
@@ -119,7 +122,7 @@ int kvmppc_mmu_map_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *orig_pte)
 
 	vpn = hpt_vpn(orig_pte->eaddr, map->host_vsid, MMU_SEGSIZE_256M);
 
-	if (!orig_pte->may_write)
+	if (!orig_pte->may_write || !writable)
 		rflags |= HPTE_R_PP;
 	else
 		mark_page_dirty(vcpu->kvm, orig_pte->raddr >> PAGE_SHIFT);
@@ -186,6 +189,17 @@ out:
 	return r;
 }
 
+void kvmppc_mmu_unmap_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *pte)
+{
+	u64 mask = 0xfffffffffULL;
+	u64 vsid;
+
+	vcpu->arch.mmu.esid_to_vsid(vcpu, pte->eaddr >> SID_SHIFT, &vsid);
+	if (vsid & VSID_64K)
+		mask = 0xffffffff0ULL;
+	kvmppc_mmu_pte_vflush(vcpu, pte->vpage, mask);
+}
+
 static struct kvmppc_sid_map *create_sid_map(struct kvm_vcpu *vcpu, u64 gvsid)
 {
 	struct kvmppc_sid_map *map;
diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index ccb89a048bf8..394fef820f0c 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -451,7 +451,7 @@ static unsigned long kvmppc_mmu_get_real_addr(unsigned long v, unsigned long r,
 }
 
 static int kvmppc_mmu_book3s_64_hv_xlate(struct kvm_vcpu *vcpu, gva_t eaddr,
-			struct kvmppc_pte *gpte, bool data)
+			struct kvmppc_pte *gpte, bool data, bool iswrite)
 {
 	struct kvm *kvm = vcpu->kvm;
 	struct kvmppc_slb *slbe;
diff --git a/arch/powerpc/kvm/book3s_pr.c b/arch/powerpc/kvm/book3s_pr.c
index 677d7e33b1ff..2f84ed807184 100644
--- a/arch/powerpc/kvm/book3s_pr.c
+++ b/arch/powerpc/kvm/book3s_pr.c
@@ -401,6 +401,7 @@ int kvmppc_handle_pagefault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 			    ulong eaddr, int vec)
 {
 	bool data = (vec == BOOK3S_INTERRUPT_DATA_STORAGE);
+	bool iswrite = false;
 	int r = RESUME_GUEST;
 	int relocated;
 	int page_found = 0;
@@ -411,10 +412,12 @@ int kvmppc_handle_pagefault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 	u64 vsid;
 
 	relocated = data ? dr : ir;
+	if (data && (vcpu->arch.fault_dsisr & DSISR_ISSTORE))
+		iswrite = true;
 
 	/* Resolve real address if translation turned on */
 	if (relocated) {
-		page_found = vcpu->arch.mmu.xlate(vcpu, eaddr, &pte, data);
+		page_found = vcpu->arch.mmu.xlate(vcpu, eaddr, &pte, data, iswrite);
 	} else {
 		pte.may_execute = true;
 		pte.may_read = true;
@@ -475,12 +478,20 @@ int kvmppc_handle_pagefault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 		kvmppc_book3s_queue_irqprio(vcpu, vec + 0x80);
 	} else if (!is_mmio &&
 		   kvmppc_visible_gfn(vcpu, pte.raddr >> PAGE_SHIFT)) {
+		if (data && !(vcpu->arch.fault_dsisr & DSISR_NOHPTE)) {
+			/*
+			 * There is already a host HPTE there, presumably
+			 * a read-only one for a page the guest thinks
+			 * is writable, so get rid of it first.
+			 */
+			kvmppc_mmu_unmap_page(vcpu, &pte);
+		}
 		/* The guest's PTE is not mapped yet. Map on the host */
-		kvmppc_mmu_map_page(vcpu, &pte);
+		kvmppc_mmu_map_page(vcpu, &pte, iswrite);
 		if (data)
 			vcpu->stat.sp_storage++;
 		else if (vcpu->arch.mmu.is_dcbz32(vcpu) &&
-			(!(vcpu->arch.hflags & BOOK3S_HFLAG_DCBZ32)))
+			 (!(vcpu->arch.hflags & BOOK3S_HFLAG_DCBZ32)))
 			kvmppc_patch_dcbz(vcpu, &pte);
 	} else {
 		/* MMIO */
@@ -732,7 +743,9 @@ int kvmppc_handle_exit(struct kvm_run *run, struct kvm_vcpu *vcpu,
 
 		/* only care about PTEG not found errors, but leave NX alone */
 		if (shadow_srr1 & 0x40000000) {
+			int idx = srcu_read_lock(&vcpu->kvm->srcu);
 			r = kvmppc_handle_pagefault(run, vcpu, kvmppc_get_pc(vcpu), exit_nr);
+			srcu_read_unlock(&vcpu->kvm->srcu, idx);
 			vcpu->stat.sp_instruc++;
 		} else if (vcpu->arch.mmu.is_dcbz32(vcpu) &&
 			  (!(vcpu->arch.hflags & BOOK3S_HFLAG_DCBZ32))) {
@@ -774,9 +787,15 @@ int kvmppc_handle_exit(struct kvm_run *run, struct kvm_vcpu *vcpu,
 		}
 #endif
 
-		/* The only case we need to handle is missing shadow PTEs */
-		if (fault_dsisr & DSISR_NOHPTE) {
+		/*
+		 * We need to handle missing shadow PTEs, and
+		 * protection faults due to us mapping a page read-only
+		 * when the guest thinks it is writable.
+		 */
+		if (fault_dsisr & (DSISR_NOHPTE | DSISR_PROTFAULT)) {
+			int idx = srcu_read_lock(&vcpu->kvm->srcu);
 			r = kvmppc_handle_pagefault(run, vcpu, dar, exit_nr);
+			srcu_read_unlock(&vcpu->kvm->srcu, idx);
 		} else {
 			vcpu->arch.shared->dar = dar;
 			vcpu->arch.shared->dsisr = fault_dsisr;