kvm: selftests: ucall: fix exit mmio address guessing

Fix two more bugs in the exit_mmio address guessing.
The first bug was that the start and step calculations were
wrong since they were dividing the number of address bits instead
of the address space. The second other bug was that the guessing
algorithm wasn't considering the valid physical and virtual address
ranges correctly for an identity map.
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/lib/ucall.c

@@ -35,6 +35,7 @@ void ucall_init(struct kvm_vm *vm, ucall_type_t type, void *arg)
 
 	if (type == UCALL_MMIO) {
 		vm_paddr_t gpa, start, end, step, offset;
+		unsigned bits;
 		bool ret;
 
 		if (arg) {
@@ -45,23 +46,30 @@ void ucall_init(struct kvm_vm *vm, ucall_type_t type, void *arg)
 		}
 
 		/*
-		 * Find an address within the allowed virtual address space,
-		 * that does _not_ have a KVM memory region associated with it.
-		 * Identity mapping an address like this allows the guest to
+		 * Find an address within the allowed physical and virtual address
+		 * spaces, that does _not_ have a KVM memory region associated with
+		 * it. Identity mapping an address like this allows the guest to
 		 * access it, but as KVM doesn't know what to do with it, it
 		 * will assume it's something userspace handles and exit with
 		 * KVM_EXIT_MMIO. Well, at least that's how it works for AArch64.
-		 * Here we start with a guess that the addresses around two
-		 * thirds of the VA space are unmapped and then work both down
-		 * and up from there in 1/12 VA space sized steps.
+		 * Here we start with a guess that the addresses around 5/8th
+		 * of the allowed space are unmapped and then work both down and
+		 * up from there in 1/16th allowed space sized steps.
+		 *
+		 * Note, we need to use VA-bits - 1 when calculating the allowed
+		 * virtual address space for an identity mapping because the upper
+		 * half of the virtual address space is the two's complement of the
+		 * lower and won't match physical addresses.
 		 */
-		start = 1ul << (vm->va_bits * 2 / 3);
-		end = 1ul << vm->va_bits;
-		step = 1ul << (vm->va_bits / 12);
+		bits = vm->va_bits - 1;
+		bits = vm->pa_bits < bits ? vm->pa_bits : bits;
+		end = 1ul << bits;
+		start = end * 5 / 8;
+		step = end / 16;
 		for (offset = 0; offset < end - start; offset += step) {
-			if (ucall_mmio_init(vm, (gpa - offset) & ~(vm->page_size - 1)))
+			if (ucall_mmio_init(vm, start - offset))
 				return;
-			if (ucall_mmio_init(vm, (gpa + offset) & ~(vm->page_size - 1)))
+			if (ucall_mmio_init(vm, start + offset))
 				return;
 		}
 		TEST_ASSERT(false, "Can't find a ucall mmio address");