[PATCH] x86: constrain sub-page access length in mmio_ro_emulated_write()

[Jan Beulich]

Without doing so we could trigger the ASSERT_UNREACHABLE() in
subpage_mmio_write_emulate(). A comment there actually says this
validation would already have been done ...

Fixes: 8847d6e23f97 ("x86/mm: add API for marking only part of a MMIO page read 
only")
Signed-off-by: Jan Beulich <jbeul...@suse.com>
---
Alternatively we could drop comment and assertion from
subpage_mmio_write_emulate().

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -5195,8 +5195,9 @@ int cf_check mmio_ro_emulated_write(
         return X86EMUL_UNHANDLEABLE;
     }
 
-    subpage_mmio_write_emulate(mmio_ro_ctxt->mfn, PAGE_OFFSET(offset),
-                               p_data, bytes);
+    if ( bytes <= 8 )
+        subpage_mmio_write_emulate(mmio_ro_ctxt->mfn, PAGE_OFFSET(offset),
+                                   p_data, bytes);
 
     return X86EMUL_OKAY;
 }