KVM: selftests: fix sync_with_host() in smm_test

It was reported that older GCCs compile smm_test in a way that breaks
it completely:

  kvm_exit:             reason EXIT_CPUID rip 0x4014db info 0 0
  func 7ffffffd idx 830 rax 0 rbx 0 rcx 0 rdx 0, cpuid entry not found
  ...
  kvm_exit:             reason EXIT_MSR rip 0x40abd9 info 0 0
  kvm_msr:              msr_read 487 = 0x0 (#GP)
  ...

Note, '7ffffffd' was supposed to be '80000001' as we're checking for
SVM. Dropping '-O2' from compiler flags help. Turns out, asm block in
sync_with_host() is wrong. We us 'in 0xe, %%al' instruction to sync
with the host and in 'AL' register we actually pass the parameter
(stage) but after sync 'AL' gets written to but GCC thinks the value
is still there and uses it to compute 'EAX' for 'cpuid'.

smm_test can't fully use standard ucall() framework as we need to
write a very simple SMI handler there. Fix the immediate issue by
making RAX input/output operand. While on it, make sync_with_host()
static inline.
Reported-by: Marcelo Bandeira Condotta <mcondotta@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200610164116.770811-1-vkuznets@redhat.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/x86_64/smm_test.c

@@ -47,10 +47,10 @@ uint8_t smi_handler[] = {
 	0x0f, 0xaa,           /* rsm */
 };
 
-void sync_with_host(uint64_t phase)
+static inline void sync_with_host(uint64_t phase)
 {
 	asm volatile("in $" XSTR(SYNC_PORT)", %%al \n"
-		     : : "a" (phase));
+		     : "+a" (phase));
 }
 
 void self_smi(void)