On Thu, May 20, 2021 at 2:27 PM David Matlack <dmatlack@xxxxxxxxxx> wrote:
>
> 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")
> Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
> ---
> tools/testing/selftests/kvm/include/kvm_util.h | 2 +-
> tools/testing/selftests/kvm/lib/kvm_util.c | 2 +-
> .../testing/selftests/kvm/lib/perf_test_util.c | 2 +-
> .../kvm/memslot_modification_stress_test.c | 18 +++++++++++-------
> 4 files changed, 14 insertions(+), 10 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/include/kvm_util.h b/tools/testing/selftests/kvm/include/kvm_util.h
> index 84982eb02b29..5d9b35d09251 100644
> --- a/tools/testing/selftests/kvm/include/kvm_util.h
> +++ b/tools/testing/selftests/kvm/include/kvm_util.h
> @@ -303,7 +303,7 @@ bool vm_is_unrestricted_guest(struct kvm_vm *vm);
>
> 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);
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index 1af1009254c4..aeffbb1e7c7d 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -2058,7 +2058,7 @@ unsigned int vm_get_page_shift(struct kvm_vm *vm)
> 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;
> }
> diff --git a/tools/testing/selftests/kvm/lib/perf_test_util.c b/tools/testing/selftests/kvm/lib/perf_test_util.c
> index 81490b9b4e32..ed4424ed26d6 100644
> --- a/tools/testing/selftests/kvm/lib/perf_test_util.c
> +++ b/tools/testing/selftests/kvm/lib/perf_test_util.c
> @@ -80,7 +80,7 @@ struct kvm_vm *perf_test_create_vm(enum vm_guest_mode mode, int vcpus,
> */
> 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: %lx max gfn: %lx vcpus: %d wss: %lx]\n",
> guest_num_pages, vm_get_max_gfn(vm), vcpus,
> vcpu_memory_bytes);
>
> diff --git a/tools/testing/selftests/kvm/memslot_modification_stress_test.c b/tools/testing/selftests/kvm/memslot_modification_stress_test.c
> index 6096bf0a5b34..98351ba0933c 100644
> --- a/tools/testing/selftests/kvm/memslot_modification_stress_test.c
> +++ b/tools/testing/selftests/kvm/memslot_modification_stress_test.c
> @@ -71,14 +71,22 @@ struct memslot_antagonist_args {
> };
>
> 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);
> }
> @@ -120,11 +128,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> 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;
>
> --
> 2.31.1.818.g46aad6cb9e-goog
Hah, good catch!
Reviewed-by: Venkatesh Srinivas <venkateshs@xxxxxxxxxxxx>
