x86emul: ignore most segment bases for 64-bit mode in is_aligned()

ops->read_segment() will report whatever is actually there in the
register, so we need to actively distinguish ES/CS/SS/DS from FS/GS.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c
index 05fa342a9400623991e204cde07bfd6a6b91aab1..dfdcd6ce7bffe1470ad43a6bbc63bff7f0bc0c26 100644 (file)
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -1642,12 +1642,17 @@ static bool is_aligned(enum x86_segment seg, unsigned long offs,
     /* Expecting powers of two only. */
     ASSERT(!(size & (size - 1)));
 
-    /* No alignment checking when we have no way to read segment data. */
-    if ( !ops->read_segment )
-        return true;
+    if ( mode_64bit() && seg < x86_seg_fs )
+        memset(&reg, 0, sizeof(reg));
+    else
+    {
+        /* No alignment checking when we have no way to read segment data. */
+        if ( !ops->read_segment )
+            return true;
 
-    if ( ops->read_segment(seg, &reg, ctxt) != X86EMUL_OKAY )
-        return false;
+        if ( ops->read_segment(seg, &reg, ctxt) != X86EMUL_OKAY )
+            return false;
+    }
 
     return !((reg.base + offs) & (size - 1));
 }