KVM: MMU: handle page removal with shadow mapping

Do not assume that a shadow mapping will always point to the same host
frame number.  Fixes crash with madvise(MADV_DONTNEED).

[avi: move after first printk(), add another printk()]
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/mmu.c

@@ -892,14 +892,25 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *shadow_pte,
 			 int *ptwrite, gfn_t gfn, struct page *page)
 {
 	u64 spte;
-	int was_rmapped = is_rmap_pte(*shadow_pte);
+	int was_rmapped = 0;
 	int was_writeble = is_writeble_pte(*shadow_pte);
+	hfn_t host_pfn = (*shadow_pte & PT64_BASE_ADDR_MASK) >> PAGE_SHIFT;
 
 	pgprintk("%s: spte %llx access %x write_fault %d"
 		 " user_fault %d gfn %lx\n",
 		 __FUNCTION__, *shadow_pte, pt_access,
 		 write_fault, user_fault, gfn);
 
+	if (is_rmap_pte(*shadow_pte)) {
+		if (host_pfn != page_to_pfn(page)) {
+			pgprintk("hfn old %lx new %lx\n",
+				 host_pfn, page_to_pfn(page));
+			rmap_remove(vcpu->kvm, shadow_pte);
+		}
+		else
+			was_rmapped = 1;
+	}
+
 	/*
 	 * We don't set the accessed bit, since we sometimes want to see
 	 * whether the guest actually used the pte (in order to detect