[xen master] x86: refine guest_mode()

[patchbot@xxxxxxx]

commit dfcf494f96671a9d3ee2fed70250ef1eedc1c00e
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Apr 9 09:12:51 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Apr 9 09:12:51 2021 +0200

    x86: refine guest_mode()
    
    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>
---
 xen/include/asm-x86/regs.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/xen/include/asm-x86/regs.h b/xen/include/asm-x86/regs.h
index 8d984ea388..3fb94deedc 100644
--- a/xen/include/asm-x86/regs.h
+++ b/xen/include/asm-x86/regs.h
@@ -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);                                    \
 })
 
 #define read_sreg(name) ({                           \
--
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