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Subject: Re: [Xen-devel] xen/mem-reservation API and out-of-tree kernel
 modules
To:     Oleksandr Andrushchenko <Oleksandr_Andrushchenko@epam.com>,
        Stefano Stabellini <sstabellini@kernel.org>
Cc:     xen-devel <xen-devel@lists.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "marmarek@invisiblethingslab.com" <marmarek@invisiblethingslab.com>
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From:   Juergen Gross <jgross@suse.com>
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Date:   Fri, 1 Feb 2019 10:14:32 +0100
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On 01/02/2019 09:39, Oleksandr Andrushchenko wrote:
> On 1/31/19 11:44 PM, Stefano Stabellini wrote:
>> On Thu, 31 Jan 2019, Oleksandr Andrushchenko wrote:
>>> 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
>> The alternative would be to turn xenmem_reservation_scrub_page into a
>> regular function (not a static inline)?
> Yes, it seems there is no other reasonable solution to this, but
> a regular function. I'll send a patch for that

What would you gain? This function would need to be exported.

So its either the variable or the function.


Juergen