[PATCH v2] x86: refine guest_mode()

[Jan Beulich <jbeulich@xxxxxxxx>]

The 2nd of the assertions as well as the macro's return value have been
assuming we're on the primary stack. While for most IST exceptions we
switch back to the main one when user mode was interrupted, for #DF we
intentionally never do, and hence a #DF actually triggering on a user
mode insn (which then is still a Xen bug) would in turn trigger this
assertion, rather than cleanly logging state.

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
v2: Correct description.
---
While we could go further and also assert we're on the correct IST
stack in an "else" ti the "if()" added, I'm not fully convinced this
would be generally helpful. I'll be happy to adjust accordingly if
others think differently; at such a point though I think this should
then no longer be a macro.

--- unstable.orig/xen/include/asm-x86/regs.h    2020-01-22 20:03:18.000000000 
+0100
+++ unstable/xen/include/asm-x86/regs.h 2020-04-27 10:02:40.009916762 +0200
@@ -10,9 +10,10 @@
     /* Frame pointer must point into current CPU stack. */                    \
     ASSERT(diff < STACK_SIZE);                                                \
     /* If not a guest frame, it must be a hypervisor frame. */                \
-    ASSERT((diff == 0) || (r->cs == __HYPERVISOR_CS));                        \
+    if ( diff < PRIMARY_STACK_SIZE )                                          \
+        ASSERT(!diff || ((r)->cs == __HYPERVISOR_CS));                        \
     /* Return TRUE if it's a guest frame. */                                  \
-    (diff == 0);                                                              \
+    !diff || ((r)->cs != __HYPERVISOR_CS);                                    \
 })
 
 #endif /* __X86_REGS_H__ */