[RFC PATCH 17/18] KVM: selftests: Add KVM Userfault mode to demand_paging_test

James Houghton
2024-07-10T23:42:55Z

The KVM Userfault mode checks that we are able to resolve KVM Userfaults
and the vCPUs will continue to make progress. It initially sets all of
guest memory as KVM_MEMORY_ATTRIBUTE_USERFAULT, then, as the test runs,
clears the attribute from pages as they are faulted on.

This test does not currently check for asynchronous page faults.

Signed-off-by: James Houghton <jthoughton@google.com>
---
.../selftests/kvm/demand_paging_test.c | 46 ++++++++++++++++++-
1 file changed, 44 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/kvm/demand_paging_test.c b/tools/testing/selftests/kvm/demand_paging_test.c
index 0202b78f8680..8654b58091b2 100644
--- a/tools/testing/selftests/kvm/demand_paging_test.c
+++ b/tools/testing/selftests/kvm/demand_paging_test.c
@@ -28,6 +28,13 @@ static uint64_t guest_percpu_mem_size = DEFAULT_PER_VCPU_MEM_SIZE;

static size_t demand_paging_size;
static char *guest_data_prototype;
+bool userfault_enabled = false;
+
+static void resolve_kvm_userfault(u64 gpa, u64 size)
+{
+ /* Toggle KVM_MEMORY_ATTRIBUTE_USERFAULT off */
+ vm_set_memory_attributes(memstress_args.vm, gpa, size, 0);
+}

static void vcpu_worker(struct memstress_vcpu_args *vcpu_args)
{
@@ -41,8 +48,22 @@ static void vcpu_worker(struct memstress_vcpu_args *vcpu_args)
clock_gettime(CLOCK_MONOTONIC, &start);

/* Let the guest access its memory */
+restart:
ret = _vcpu_run(vcpu);
- TEST_ASSERT(ret == 0, "vcpu_run failed: %d", ret);
+ if (ret < 0 && errno == EFAULT && userfault_enabled) {
+ /* Check for userfault. */
+ TEST_ASSERT(run->exit_reason == KVM_EXIT_MEMORY_FAULT,
+ "Got invalid exit reason: %llx", run->exit_reason);
+ TEST_ASSERT(run->memory_fault.flags ==
+ KVM_MEMORY_EXIT_FLAG_USERFAULT,
+ "Got invalid memory fault exit: %llx",
+ run->memory_fault.flags);
+ resolve_kvm_userfault(run->memory_fault.gpa,
+ run->memory_fault.size);
+ goto restart;
+ } else
+ TEST_ASSERT(ret == 0, "vcpu_run failed: %d", ret);
+
if (get_ucall(vcpu, NULL) != UCALL_SYNC) {
TEST_ASSERT(false,
"Invalid guest sync status: exit_reason=%s",
@@ -136,6 +157,7 @@ struct test_params {
int readers_per_uffd;
enum vm_mem_backing_src_type src_type;
bool partition_vcpu_memory_access;
+ bool kvm_userfault;
};

static void prefault_mem(void *alias, uint64_t len)
@@ -206,6 +228,17 @@ static void run_test(enum vm_guest_mode mode, void *arg)
}
}

+ if (p->kvm_userfault) {
+ TEST_REQUIRE(kvm_has_cap(KVM_CAP_USERFAULT));
+ vm_enable_cap(vm, KVM_CAP_USERFAULT, KVM_USERFAULT_ENABLE);
+ TEST_REQUIRE(kvm_check_cap(KVM_CAP_MEMORY_ATTRIBUTES) &
+ KVM_MEMORY_ATTRIBUTE_USERFAULT);
+ vm_set_memory_attributes(vm, memstress_args.gpa,
+ memstress_args.size,
+ KVM_MEMORY_ATTRIBUTE_USERFAULT);
+ userfault_enabled = true;
+ }
+
pr_info("Finished creating vCPUs and starting uffd threads\n");

clock_gettime(CLOCK_MONOTONIC, &start);
@@ -232,6 +265,11 @@ static void run_test(enum vm_guest_mode mode, void *arg)
pr_info("Overall demand paging rate:\t%f pgs/sec\n",
vcpu_paging_rate * nr_vcpus);

+ if (p->kvm_userfault) {
+ vm_enable_cap(vm, KVM_CAP_USERFAULT, KVM_USERFAULT_DISABLE);
+ userfault_enabled = false;
+ }
+
memstress_destroy_vm(vm);

free(guest_data_prototype);
@@ -263,6 +301,7 @@ static void help(char *name)
printf(" -v: specify the number of vCPUs to run.\n");
printf(" -o: Overlap guest memory accesses instead of partitioning\n"
" them into a separate region of memory for each vCPU.\n");
+ printf(" -k: Use KVM Userfault\n");
puts("");
exit(0);
}
@@ -281,7 +320,7 @@ int main(int argc, char *argv[])

guest_modes_append_default();

- while ((opt = getopt(argc, argv, "ahom:u:d:b:s:v:c:r:")) != -1) {
+ while ((opt = getopt(argc, argv, "ahokm:u:d:b:s:v:c:r:")) != -1) {
switch (opt) {
case 'm':
guest_modes_cmdline(optarg);
@@ -324,6 +363,9 @@ int main(int argc, char *argv[])
"Invalid number of readers per uffd %d: must be >=1",
p.readers_per_uffd);
break;
+ case 'k':
+ p.kvm_userfault = true;
+ break;
case 'h':
default:
help(argv[0]);
--
2.45.2.993.g49e7a77208-goog