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

On Thu, Oct 15, 2015 at 12:24:39PM +0100, David Vrabel wrote:

On 15/10/15 12:18, Mikko Rapeli wrote:

quoted

On Thu, Oct 15, 2015 at 11:29:12AM +0100, David Vrabel wrote:

quoted

On 15/10/15 06:56, Mikko Rapeli wrote:

quoted

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 [off-list ref] on lkml message
[off-list ref].

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

[...]

quoted

--- 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

quoted

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.

There are not many headers in uapi which require additional userspace
headers to compile (I'm not saying that uapi headers represent the full
API's/ABI's, it's clear they don't). If there are ~700 headers and just a
single one needs an exception and additional dependencies, I would try to
make it work without the dependency. I just need some advice and guidance
from maintainers how to do this.

Or we should start tracking these userspace library dependencies too,
at least at linux-headers package levels in distributions. This might
be a good idea anyway, but then we might as well move headers like this
directly to the userspace library and out of include/uapi.

Both would solve the non-compiling uapi header problem.

quoted

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).

Understood.

-Mikko

David