KVM: MMU: Keep a reverse mapping of non-writable translations

The current kvm mmu only reverse maps writable translation.  This is used
to write-protect a page in case it becomes a pagetable.

But with swapping support, we need a reverse mapping of read-only pages as
well:  when we evict a page, we need to remove any mapping to it, whether
writable or not.
Signed-off-by: NIzik Eidus <izike@qumranet.com>
Signed-off-by: NAvi Kivity <avi@qumranet.com>

drivers/kvm/mmu.c

@@ -211,8 +211,8 @@ static int is_io_pte(unsigned long pte)
 
 static int is_rmap_pte(u64 pte)
 {
-	return (pte & (PT_WRITABLE_MASK | PT_PRESENT_MASK))
-		== (PT_WRITABLE_MASK | PT_PRESENT_MASK);
+	return pte != shadow_trap_nonpresent_pte
+		&& pte != shadow_notrap_nonpresent_pte;
 }
 
 static void set_shadow_pte(u64 *sptep, u64 spte)
@@ -488,7 +488,6 @@ static void rmap_write_protect(struct kvm *kvm, u64 gfn)
 {
 	unsigned long *rmapp;
 	u64 *spte;
-	u64 *prev_spte;
 
 	gfn = unalias_gfn(kvm, gfn);
 	rmapp = gfn_to_rmap(kvm, gfn);
@@ -497,13 +496,11 @@ static void rmap_write_protect(struct kvm *kvm, u64 gfn)
 	while (spte) {
 		BUG_ON(!spte);
 		BUG_ON(!(*spte & PT_PRESENT_MASK));
-		BUG_ON(!(*spte & PT_WRITABLE_MASK));
 		rmap_printk("rmap_write_protect: spte %p %llx\n", spte, *spte);
-		prev_spte = spte;
-		spte = rmap_next(kvm, rmapp, spte);
-		rmap_remove(kvm, prev_spte);
-		set_shadow_pte(prev_spte, *prev_spte & ~PT_WRITABLE_MASK);
+		if (is_writeble_pte(*spte))
+			set_shadow_pte(spte, *spte & ~PT_WRITABLE_MASK);
 		kvm_flush_remote_tlbs(kvm);
+		spte = rmap_next(kvm, rmapp, spte);
 	}
 }
 
@@ -908,14 +905,18 @@ static int nonpaging_map(struct kvm_vcpu *vcpu, gva_t v, hpa_t p)
 		table = __va(table_addr);
 
 		if (level == 1) {
+			int was_rmapped;
+
 			pte = table[index];
+			was_rmapped = is_rmap_pte(pte);
 			if (is_shadow_present_pte(pte) && is_writeble_pte(pte))
 				return 0;
 			mark_page_dirty(vcpu->kvm, v >> PAGE_SHIFT);
 			page_header_update_slot(vcpu->kvm, table, v);
 			table[index] = p | PT_PRESENT_MASK | PT_WRITABLE_MASK |
 								PT_USER_MASK;
-			rmap_add(vcpu, &table[index], v >> PAGE_SHIFT);
+			if (!was_rmapped)
+				rmap_add(vcpu, &table[index], v >> PAGE_SHIFT);
 			return 0;
 		}
 
@@ -1424,10 +1425,8 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
 		pt = page->spt;
 		for (i = 0; i < PT64_ENT_PER_PAGE; ++i)
 			/* avoid RMW */
-			if (pt[i] & PT_WRITABLE_MASK) {
-				rmap_remove(kvm, &pt[i]);
+			if (pt[i] & PT_WRITABLE_MASK)
 				pt[i] &= ~PT_WRITABLE_MASK;
-			}
 	}
 }