From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
[ Upstream commit ef4059809890f732c69cc1726d3a9a108a832a2f ]
Commit 204c91eff798a ("KVM: selftests: do not blindly clobber registers in
guest asm") was intended to make test more gcc-proof, however, the result
is exactly the opposite: on newer gccs (e.g. 8.2.1) the test breaks with
==== Test Assertion Failure ====
x86_64/sync_regs_test.c:168: run->s.regs.regs.rbx == 0xBAD1DEA + 1
pid=14170 tid=14170 - Invalid argument
1 0x00000000004015b3: main at sync_regs_test.c:166 (discriminator 6)
2 0x00007f413fb66412: ?? ??:0
3 0x000000000040191d: _start at ??:?
rbx sync regs value incorrect 0x1.
Apparently, compile is still free to play games with registers even
when they have variables attached.
Re-write guest code with 'asm volatile' by embedding ucall there and
making sure rbx is preserved.
Fixes: 204c91eff798a ("KVM: selftests: do not blindly clobber registers in guest asm")
Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
.../selftests/kvm/x86_64/sync_regs_test.c | 21 ++++++++++---------
1 file changed, 11 insertions(+), 10 deletions(-)
diff --git a/tools/testing/selftests/kvm/x86_64/sync_regs_test.c b/tools/testing/selftests/kvm/x86_64/sync_regs_test.c
index 11c2a70a7b87a..5c82242562943 100644
--- a/tools/testing/selftests/kvm/x86_64/sync_regs_test.c
+++ b/tools/testing/selftests/kvm/x86_64/sync_regs_test.c
@@ -22,18 +22,19 @@
#define VCPU_ID 5
+#define UCALL_PIO_PORT ((uint16_t)0x1000)
+
+/*
+ * ucall is embedded here to protect against compiler reshuffling registers
+ * before calling a function. In this test we only need to get KVM_EXIT_IO
+ * vmexit and preserve RBX, no additional information is needed.
+ */
void guest_code(void)
{
- /*
- * use a callee-save register, otherwise the compiler
- * saves it around the call to GUEST_SYNC.
- */
- register u32 stage asm("rbx");
- for (;;) {
- GUEST_SYNC(0);
- stage++;
- asm volatile ("" : : "r" (stage));
- }
+ asm volatile("1: in %[port], %%al\n"
+ "add $0x1, %%rbx\n"
+ "jmp 1b"
+ : : [port] "d" (UCALL_PIO_PORT) : "rax", "rbx");
}
static void compare_regs(struct kvm_regs *left, struct kvm_regs *right)
--
2.20.1
