KVM: selftests: Add kvm_gettid() wrapper and convert users

Add a KVM wrapper for the gettid() syscall so that tests don't have to
open code the syscall() themselves.  Unfortunately, not all flavors of
libc that KVM selftests support provide gettid().  Convert all existing
users of the syscall to the new wrapper.

Note, per the gettid() manpage[1], "This call is always successful", i.e.
prefixing kvm_ to the syscall name is aligned with the goal of providing
syscall wrappers that guarantee success.

No functional changes intended.

Link: https://man7.org/linux/man-pages/man2/gettid.2.html [1]
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Josh Hilke <jrhilke@google.com>
[sean: massage changelog]
Link: https://patch.msgid.link/20260626213534.3866178-16-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/demand_paging_test.c

@@ -57,7 +57,7 @@ static void vcpu_worker(struct memstress_vcpu_args *vcpu_args)
 static int handle_uffd_page_request(int uffd_mode, int uffd,
 		struct uffd_msg *msg)
 {
-	pid_t tid = syscall(__NR_gettid);
+	pid_t tid = kvm_gettid();
 	u64 addr = msg->arg.pagefault.address;
 	struct timespec start;
 	struct timespec ts_diff;

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

@@ -83,6 +83,11 @@ static inline int kvm_dup(int fd)
 	return new_fd;
 }
 
+static inline pid_t kvm_gettid(void)
+{
+	return syscall(__NR_gettid);
+}
+
 __KVM_SYSCALL_DEFINE(munmap, 2, void *, mem, size_t, size);
 __KVM_SYSCALL_DEFINE(close, 1, int, fd);
 __KVM_SYSCALL_DEFINE(fallocate, 4, int, fd, int, mode, loff_t, offset, loff_t, len);

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

@@ -10,6 +10,7 @@
 #include <sys/syscall.h>
 
 #include "kselftest.h"
+#include "kvm_syscalls.h"
 
 #ifdef __GLIBC__
 #include <execinfo.h>
@@ -64,11 +65,6 @@ static void test_dump_stack(void)
 static void test_dump_stack(void) {}
 #endif
 
-static pid_t _gettid(void)
-{
-	return syscall(SYS_gettid);
-}
-
 void __attribute__((noinline))
 test_assert(bool exp, const char *exp_str,
 	const char *file, unsigned int line, const char *fmt, ...)
@@ -81,7 +77,7 @@ test_assert(bool exp, const char *exp_str,
 		fprintf(stderr, "==== Test Assertion Failure ====\n"
 			"  %s:%u: %s\n"
 			"  pid=%d tid=%d errno=%d - %s\n",
-			file, line, exp_str, getpid(), _gettid(),
+			file, line, exp_str, getpid(), kvm_gettid(),
 			errno, strerror(errno));
 		test_dump_stack();
 		if (fmt) {

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

@@ -17,6 +17,7 @@
 #include "linux/kernel.h"
 
 #include "test_util.h"
+#include "kvm_syscalls.h"
 
 sigjmp_buf expect_sigbus_jmpbuf;
 
@@ -395,7 +396,7 @@ long get_run_delay(void)
 	long val[2];
 	FILE *fp;
 
-	sprintf(path, "/proc/%ld/schedstat", syscall(SYS_gettid));
+	sprintf(path, "/proc/%ld/schedstat", (long)kvm_gettid());
 	fp = fopen(path, "r");
 	/* Return MIN_RUN_DELAY_NS upon failure just to be safe */
 	if (fscanf(fp, "%ld %ld ", &val[0], &val[1]) < 2)

tools/testing/selftests/kvm/rseq_test.c

@@ -244,7 +244,7 @@ int main(int argc, char *argv[])
 	vm = vm_create_with_one_vcpu(&vcpu, guest_code);
 
 	pthread_create(&migration_thread, NULL, migration_worker,
-		       (void *)(unsigned long)syscall(SYS_gettid));
+		       (void *)(unsigned long)kvm_gettid());
 
 	if (latency >= 0) {
 		/*