[PATCH MANUALSEL 5.4] selftests: KVM: Handle compiler optimizations in ucall

[Sasha Levin]

From: Raghavendra Rao Ananta <rananta at google.com>

[ Upstream commit 9e2f6498efbbc880d7caa7935839e682b64fe5a6 ]

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 at google.com>
Suggested-by: Reiji Watanabe <reijiw at google.com>
Signed-off-by: Raghavendra Rao Ananta <rananta at google.com>
Message-Id: <20220615185706.1099208-1-rananta at google.com>
Reviewed-by: Andrew Jones <drjones at redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
 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 6cd91970fbad..3b2a426070c4 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)
-- 
2.35.1