[PATCH RFC] x86: Fix XEN_DOMCTL_gdbsx_guestmemio crash

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

When CONFIG_GDBSX is compiled out, iommu_do_domctl() falls over a NULL
pointer.  It isn't really correct for processing of XEN_DOMCTL_gdbsx_* to fall
into the default case when compiled out.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>

RFC, because this has implications across the codebase.  The tl;dr is that
case FOO:'s shouldn't be compiled out; we still know what the subops are, even
when the functionality is compiled out.

There are several ways to express this.  Alternatives would be:

    case XEN_DOMCTL_gdbsx_guestmemio:
        if ( !IS_ENABLED(CONFIG_GDBSX) )
        {
            rc = -EOPNOTSUPP;
            break;
        }
        ...;

but given my debugger series creating gdbsx.c, I was also considering:

    case XEN_DOMCTL_gdbsx_guestmemio:
    case XEN_DOMCTL_gdbsx_pausevcpu:
    case XEN_DOMCTL_gdbsx_unpausevcpu:
    case XEN_DOMCTL_gdbsx_domstatus:
        rc = gdbsx_do_domctl(d, iop);
        break;

when I can rework the callers of domain_pause_for_debugger() slightly, at
which point we can conditionally compile the gdbsx variables out struct
domain/vcpu, which wouldn't be compatible with the first suggestion.

Thoughts?
---
 xen/arch/x86/domctl.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
index a6aae500a30b..1faa5a49ff3c 100644
--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -890,7 +890,14 @@ long arch_do_domctl(
         copyback = true;
         break;
     }
-#endif
+#else /* CONFIG_GDBSX */
+    case XEN_DOMCTL_gdbsx_guestmemio:
+    case XEN_DOMCTL_gdbsx_pausevcpu:
+    case XEN_DOMCTL_gdbsx_unpausevcpu:
+    case XEN_DOMCTL_gdbsx_domstatus:
+        rc = -EOPNOTSUPP;
+        break;
+#endif /* CONFIG_GDBSX */
 
     case XEN_DOMCTL_setvcpuextstate:
     case XEN_DOMCTL_getvcpuextstate:
-- 
2.11.0