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Subject[Xen-devel] xen/mem-reservation API and out-of-tree kernel modules
Date
Hello,

I am working on porting an out-of-tree kernel driver to the kernel
5.0 and that driver uses functionality provided by
drivers/xen/mem-reservation.c
module.  Since commit [1] it is not possible to build a kernel module
which uses mem-reservation API as xen_scrub_pages variable, which is
checked in
xenmem_reservation_scrub_page, became a kernel module parameter and is
now only
accessible for built-in modules:

static inline void xenmem_reservation_scrub_page(struct page *page)
^^^^^^^^^^^^^
{
    if (xen_scrub_pages)
        ^^^^^^^^^^^^^^^
        clear_highpage(page);
}

This results in link-time warning:

    WARNING: "xen_scrub_pages" [yourmodule.ko] undefined!

and thus not allowing the module to run. At the moment I can only see a
possible fix
for this by making the following change:

diff --git a/drivers/xen/mem-reservation.c b/drivers/xen/mem-reservation.c
index 3782cf070338..85fecfec50e1 100644
--- a/drivers/xen/mem-reservation.c
+++ b/drivers/xen/mem-reservation.c
@@ -18,6 +18,7 @@

 bool __read_mostly xen_scrub_pages =
IS_ENABLED(CONFIG_XEN_SCRUB_PAGES_DEFAULT);
 core_param(xen_scrub_pages, xen_scrub_pages, bool, 0);
+EXPORT_SYMBOL(xen_scrub_pages);

but this looks a bit unusual for the kernel?

I am looking for community advice here and help

Thank you,
Oleksandr

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=197ecb3802c04499d8ff4f8cb28f6efa008067db
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