KVM: selftests: Fix 32-bit truncation of vm_get_max_gfn()

vm_get_max_gfn() casts vm->max_gfn from a uint64_t to an unsigned int,
which causes the upper 32-bits of the max_gfn to get truncated.

Nobody noticed until now likely because vm_get_max_gfn() is only used
as a mechanism to create a memslot in an unused region of the guest
physical address space (the top), and the top of the 32-bit physical
address space was always good enough.

This fix reveals a bug in memslot_modification_stress_test which was
trying to create a dummy memslot past the end of guest physical memory.
Fix that by moving the dummy memslot lower.

Fixes: 52200d0d944e ("KVM: selftests: Remove duplicate guest mode handling")
Reviewed-by: Venkatesh Srinivas <[e-mail]>
Signed-off-by: David Matlack <[e-mail]>
Message-Id: <[e-mail]>
Reviewed-by: Andrew Jones <[e-mail]>
Reviewed-by: Peter Xu <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

tools/testing/selftests/kvm/include/kvm_util.h

···
 
 unsigned int vm_get_page_size(struct kvm_vm *vm);
 unsigned int vm_get_page_shift(struct kvm_vm *vm);
-unsigned int vm_get_max_gfn(struct kvm_vm *vm);
+uint64_t vm_get_max_gfn(struct kvm_vm *vm);
 int vm_get_fd(struct kvm_vm *vm);
 
 unsigned int vm_calc_num_guest_pages(enum vm_guest_mode mode, size_t size);

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

···
 	return vm->page_shift;
 }
 
-unsigned int vm_get_max_gfn(struct kvm_vm *vm)
+uint64_t vm_get_max_gfn(struct kvm_vm *vm)
 {
 	return vm->max_gfn;
 }

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

···
 /*
  * Copyright (C) 2020, Google LLC.
  */
+#include <inttypes.h>
 
 #include "kvm_util.h"
 #include "perf_test_util.h"
···
 	 */
 	TEST_ASSERT(guest_num_pages < vm_get_max_gfn(vm),
 		    "Requested more guest memory than address space allows.\n"
-		    "    guest pages: %lx max gfn: %x vcpus: %d wss: %lx]\n",
+		    "    guest pages: %" PRIx64 " max gfn: %" PRIx64
+		    " vcpus: %d wss: %" PRIx64 "]\n",
 		    guest_num_pages, vm_get_max_gfn(vm), vcpus,
 		    vcpu_memory_bytes);
 

tools/testing/selftests/kvm/memslot_modification_stress_test.c

···
 };
 
 static void add_remove_memslot(struct kvm_vm *vm, useconds_t delay,
-			      uint64_t nr_modifications, uint64_t gpa)
+			       uint64_t nr_modifications)
 {
+	const uint64_t pages = 1;
+	uint64_t gpa;
 	int i;
+
+	/*
+	 * Add the dummy memslot just below the perf_test_util memslot, which is
+	 * at the top of the guest physical address space.
+	 */
+	gpa = guest_test_phys_mem - pages * vm_get_page_size(vm);
 
 	for (i = 0; i < nr_modifications; i++) {
 		usleep(delay);
 		vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS, gpa,
-					    DUMMY_MEMSLOT_INDEX, 1, 0);
+					    DUMMY_MEMSLOT_INDEX, pages, 0);
 
 		vm_mem_region_delete(vm, DUMMY_MEMSLOT_INDEX);
 	}
···
 	pr_info("Started all vCPUs\n");
 
 	add_remove_memslot(vm, p->memslot_modification_delay,
-			   p->nr_memslot_modifications,
-			   guest_test_phys_mem +
-			   (guest_percpu_mem_size * nr_vcpus) +
-			   perf_test_args.host_page_size +
-			   perf_test_args.guest_page_size);
+			   p->nr_memslot_modifications);
 
 	run_vcpus = false;