xen: set vma flag VM_PFNMAP in the privcmd mmap file_op

Set VM_PFNMAP in the privcmd mmap file_op, rather than later in
xen_remap_domain_mfn_range when it is too late because
vma_wants_writenotify has already been called and vm_page_prot has
already been modified.
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/mmu.c

@@ -2299,7 +2299,8 @@ int xen_remap_domain_mfn_range(struct vm_area_struct *vma,
 
 	prot = __pgprot(pgprot_val(prot) | _PAGE_IOMAP);
 
-	vma->vm_flags |= VM_IO | VM_RESERVED | VM_PFNMAP;
+	BUG_ON(!((vma->vm_flags & (VM_PFNMAP | VM_RESERVED | VM_IO)) ==
+				(VM_PFNMAP | VM_RESERVED | VM_IO)));
 
 	rmd.mfn = mfn;
 	rmd.prot = prot;

drivers/xen/xenfs/privcmd.c

@@ -380,8 +380,9 @@ static int privcmd_mmap(struct file *file, struct vm_area_struct *vma)
 	if (xen_feature(XENFEAT_auto_translated_physmap))
 		return -ENOSYS;
 
-	/* DONTCOPY is essential for Xen as copy_page_range is broken. */
-	vma->vm_flags |= VM_RESERVED | VM_IO | VM_DONTCOPY;
+	/* DONTCOPY is essential for Xen because copy_page_range doesn't know
+	 * how to recreate these mappings */
+	vma->vm_flags |= VM_RESERVED | VM_IO | VM_DONTCOPY | VM_PFNMAP;
 	vma->vm_ops = &privcmd_vm_ops;
 	vma->vm_private_data = NULL;