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Re: [Xen-devel] xen/mem-reservation API and out-of-tree kernel modules


  • To: Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
  • From: Juergen Gross <jgross@xxxxxxxx>
  • Date: Fri, 1 Feb 2019 10:14:32 +0100
  • Autocrypt: addr=jgross@xxxxxxxx; prefer-encrypt=mutual; keydata= xsBNBFOMcBYBCACgGjqjoGvbEouQZw/ToiBg9W98AlM2QHV+iNHsEs7kxWhKMjrioyspZKOB ycWxw3ie3j9uvg9EOB3aN4xiTv4qbnGiTr3oJhkB1gsb6ToJQZ8uxGq2kaV2KL9650I1SJve dYm8Of8Zd621lSmoKOwlNClALZNew72NjJLEzTalU1OdT7/i1TXkH09XSSI8mEQ/ouNcMvIJ NwQpd369y9bfIhWUiVXEK7MlRgUG6MvIj6Y3Am/BBLUVbDa4+gmzDC9ezlZkTZG2t14zWPvx XP3FAp2pkW0xqG7/377qptDmrk42GlSKN4z76ELnLxussxc7I2hx18NUcbP8+uty4bMxABEB AAHNHkp1ZXJnZW4gR3Jvc3MgPGpncm9zc0BzdXNlLmRlPsLAeQQTAQIAIwUCU4xw6wIbAwcL CQgHAwIBBhUIAgkKCwQWAgMBAh4BAheAAAoJELDendYovxMvi4UH/Ri+OXlObzqMANruTd4N zmVBAZgx1VW6jLc8JZjQuJPSsd/a+bNr3BZeLV6lu4Pf1Yl2Log129EX1KWYiFFvPbIiq5M5 kOXTO8Eas4CaScCvAZ9jCMQCgK3pFqYgirwTgfwnPtxFxO/F3ZcS8jovza5khkSKL9JGq8Nk czDTruQ/oy0WUHdUr9uwEfiD9yPFOGqp4S6cISuzBMvaAiC5YGdUGXuPZKXLpnGSjkZswUzY d9BVSitRL5ldsQCg6GhDoEAeIhUC4SQnT9SOWkoDOSFRXZ+7+WIBGLiWMd+yKDdRG5RyP/8f 3tgGiB6cyuYfPDRGsELGjUaTUq3H2xZgIPfOwE0EU4xwFgEIAMsx+gDjgzAY4H1hPVXgoLK8 B93sTQFN9oC6tsb46VpxyLPfJ3T1A6Z6MVkLoCejKTJ3K9MUsBZhxIJ0hIyvzwI6aYJsnOew cCiCN7FeKJ/oA1RSUemPGUcIJwQuZlTOiY0OcQ5PFkV5YxMUX1F/aTYXROXgTmSaw0aC1Jpo w7Ss1mg4SIP/tR88/d1+HwkJDVW1RSxC1PWzGizwRv8eauImGdpNnseneO2BNWRXTJumAWDD pYxpGSsGHXuZXTPZqOOZpsHtInFyi5KRHSFyk2Xigzvh3b9WqhbgHHHE4PUVw0I5sIQt8hJq 5nH5dPqz4ITtCL9zjiJsExHuHKN3NZsAEQEAAcLAXwQYAQIACQUCU4xwFgIbDAAKCRCw3p3W KL8TL0P4B/9YWver5uD/y/m0KScK2f3Z3mXJhME23vGBbMNlfwbr+meDMrJZ950CuWWnQ+d+ Ahe0w1X7e3wuLVODzjcReQ/v7b4JD3wwHxe+88tgB9byc0NXzlPJWBaWV01yB2/uefVKryAf AHYEd0gCRhx7eESgNBe3+YqWAQawunMlycsqKa09dBDL1PFRosF708ic9346GLHRc6Vj5SRA UTHnQqLetIOXZm3a2eQ1gpQK9MmruO86Vo93p39bS1mqnLLspVrL4rhoyhsOyh0Hd28QCzpJ wKeHTd0MAWAirmewHXWPco8p1Wg+V+5xfZzuQY0f4tQxvOpXpt4gQ1817GQ5/Ed/wsDtBBgB CAAgFiEEhRJncuj2BJSl0Jf3sN6d1ii/Ey8FAlrd8NACGwIAgQkQsN6d1ii/Ey92IAQZFggA HRYhBFMtsHpB9jjzHji4HoBcYbtP2GO+BQJa3fDQAAoJEIBcYbtP2GO+TYsA/30H/0V6cr/W V+J/FCayg6uNtm3MJLo4rE+o4sdpjjsGAQCooqffpgA+luTT13YZNV62hAnCLKXH9n3+ZAgJ RtAyDWk1B/0SMDVs1wxufMkKC3Q/1D3BYIvBlrTVKdBYXPxngcRoqV2J77lscEvkLNUGsu/z W2pf7+P3mWWlrPMJdlbax00vevyBeqtqNKjHstHatgMZ2W0CFC4hJ3YEetuRBURYPiGzuJXU pAd7a7BdsqWC4o+GTm5tnGrCyD+4gfDSpkOT53S/GNO07YkPkm/8J4OBoFfgSaCnQ1izwgJQ jIpcG2fPCI2/hxf2oqXPYbKr1v4Z1wthmoyUgGN0LPTIm+B5vdY82wI5qe9uN6UOGyTH2B3p hRQUWqCwu2sqkI3LLbTdrnyDZaixT2T0f4tyF5Lfs+Ha8xVMhIyzNb1byDI5FKCb
  • Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, "marmarek@xxxxxxxxxxxxxxxxxxxxxx" <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Fri, 01 Feb 2019 09:14:44 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

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

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