linux-kernel - Re: [PATCH v4 75/79] include/uapi/xen/privcmd.h: fix compilation in userspace

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Date:	Thu, 15 Oct 2015 12:24:39 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Mikko Rapeli <mikko.rapeli@....fi>
CC:	<linux-kernel@...r.kernel.org>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Russell King <linux@....linux.org.uk>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<xen-devel@...ts.xenproject.org>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-api@...r.kernel.org>
Subject: Re: [PATCH v4 75/79] include/uapi/xen/privcmd.h: fix compilation
 in userspace

On 15/10/15 12:18, Mikko Rapeli wrote:
> On Thu, Oct 15, 2015 at 11:29:12AM +0100, David Vrabel wrote:
>> On 15/10/15 06:56, Mikko Rapeli wrote:
>>> xen/interface/xen.h is not exported from kernel headers so remove the
>>> dependency and provide needed defines for domid_t and xen_pfn_t if they
>>> are not already defined by some other e.g. Xen specific headers.
>>>
>>> Suggested by Andrew Cooper <andrew.cooper3@...rix.com> on lkml message
>>> <5569F9C9.8000607@...rix.com>.
>>>
>>> The ifdef for ARM is ugly but did not find better solutions for it.
>>>
>>> Fixes userspace compilation error:
>>>
>>> xen/privcmd.h:38:31: fatal error: xen/interface/xen.h: No such file or directory
>> [...]
>>> --- a/include/uapi/xen/privcmd.h
>>> +++ b/include/uapi/xen/privcmd.h
>>> @@ -35,7 +35,19 @@
>>>  
>>>  #include <linux/types.h>
>>>  #include <linux/compiler.h>
>>> -#include <xen/interface/xen.h>
>>> +
>>> +/* Might be defined by Xen specific headers, but if not */
>>> +#ifndef domid_t
>>> +typedef __u16 domid_t;
>>> +#endif /* domid_t */
>>
>> As the kbuild bot points out, this does not work since the existence of
>> a typedef cannot be checked with #ifdef.
> 
> Yeah, this hack doesn't cut it. Sorry. Tried to implement these changes:
> http://www.spinics.net/lists/linux-api/msg11048.html
> 
>> I'm not really sure what problem you're trying to solve.  A user space
>> program making use of this interface gets the domid_t and xen_pfn_t etc
>> typedefs from the headers provided as part of the libxenctrl library.
> 
> I'm trying to make sure that kernel headers in userspace compile with minimal
> dependencies which are gcc and libc.
> 
> For me it is clear by now that many Linux API's and ABI's like Xen parts do
> not live in the uapi header files and instead there's a separate userspace
> library with needed headers and defines which have embedded copies of
> the needed API and ABI definitions, like header files.
> 
> So how could this file be changed so that it compiles in userspace without
> definitions from libxenctrl?

I don't think anything needs to be changed.

Instead I would make your compilation check of this header dependent on
the existence of the xen/interface/xen.h header. Or you may exclude the
check of this header entirely.

> I guess I could copy the needed definitions for domid_t and xen_pfn_t from
> xen/interface/xen.h of libxenctrl. That I should have done to begin with
> instead of trying to hack something on my own.

I do not want definitions duplicated/copied from the hypervisor ABI
headers.  This causes problems when support for new architectures is
added (for example).

David
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