selftests: kvm: fix sync_regs_test with newer gccs

Commit 204c91e ("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: 204c91e ("KVM: selftests: do not blindly clobber registers in guest asm")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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)