[PATCH] selftests: KVM: Handle compiler optimizations in ucall

[Raghavendra Rao Ananta]

The selftests, when built with newer versions of clang, is found
to have over optimized guests' ucall() function, and eliminating
the stores for uc.cmd (perhaps due to no immediate readers). This
resulted in the userspace side always reading a value of '0', and
causing multiple test failures.

As a result, prevent the compiler from optimizing the stores in
ucall() with WRITE_ONCE().

Suggested-by: Ricardo Koller <ricarkol@xxxxxxxxxx>
Suggested-by: Reiji Watanabe <reijiw@xxxxxxxxxx>
Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxx>
---
 tools/testing/selftests/kvm/lib/aarch64/ucall.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/kvm/lib/aarch64/ucall.c b/tools/testing/selftests/kvm/lib/aarch64/ucall.c
index e0b0164e9af8..be1d9728c4ce 100644
--- a/tools/testing/selftests/kvm/lib/aarch64/ucall.c
+++ b/tools/testing/selftests/kvm/lib/aarch64/ucall.c
@@ -73,20 +73,19 @@ void ucall_uninit(struct kvm_vm *vm)
 
 void ucall(uint64_t cmd, int nargs, ...)
 {
-	struct ucall uc = {
-		.cmd = cmd,
-	};
+	struct ucall uc = {};
 	va_list va;
 	int i;
 
+	WRITE_ONCE(uc.cmd, cmd);
 	nargs = nargs <= UCALL_MAX_ARGS ? nargs : UCALL_MAX_ARGS;
 
 	va_start(va, nargs);
 	for (i = 0; i < nargs; ++i)
-		uc.args[i] = va_arg(va, uint64_t);
+		WRITE_ONCE(uc.args[i], va_arg(va, uint64_t));
 	va_end(va);
 
-	*ucall_exit_mmio_addr = (vm_vaddr_t)&uc;
+	WRITE_ONCE(*ucall_exit_mmio_addr, (vm_vaddr_t)&uc);
 }
 
 uint64_t get_ucall(struct kvm_vm *vm, uint32_t vcpu_id, struct ucall *uc)
-- 
2.36.1.476.g0c4daa206d-goog