Re: [PATCH v2 1/2] vfio: fix build if build envrionment is on old kernel

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2015-07-10 13:44, Stephen Hemminger:
> On Fri, 10 Jul 2015 22:19:58 +0200
> Thomas Monjalon <thomas.monjalon@6wind.com> wrote:
> 
> > 2015-07-10 10:29, Stephen Hemminger:
> > > On Fri, 10 Jul 2015 01:43:17 +0200
> > > Thomas Monjalon <thomas.monjalon@6wind.com> wrote:
> > > 
> > > > 2015-07-09 16:26, Stephen Hemminger:
> > > > > From: Stephen Hemminger <shemming@brocade.com>
> > > > > 
> > > > > The build of DPDK may be done on a system where Linux headers
> > > > > in /usr/include (and therefore kernel version macro) are much
> > > > > older than the target runtime system.
> > > > 
> > > > It seems strange wanting to build a feature not present in the kernel
> > > > headers. Why not upgrading the build system kernel?
> > > 
> > > The build machine is running Debian with stock headers and kernel.
> > > I can see many people having build environments controlled by other
> > > parts of organization where you are not allowed to update packages.
> > > 
> > > > > In order to work around this, one solution is to put in simplified
> > > > > kernel header (this is what the compat stuff is already doing).
> > > > 
> > > > The other solution (as already suggested by Anatoly) is to have a configure
> > > > script (not an autotool one). It would make clear that VFIO support is not
> > > > built.
> > > > Ref: http://dpdk.org/ml/archives/dev/2015-April/016772.html
> > > 
> > > That won't solve the issue.
> > 
> > That allows to see the error earlier and more clearly.
> > 
> > > The other possiblities are:
> > >  1. Fail the build if VFIO is configured but headers files are missing.
> > >     This is better than surprising user at runtime.
> > >  2. Don't use /usr/include/linux as path as expected path of kernel headers
> > >     instead use build path (this is what modules do).
> > 
> > The latter one seems to be the right one. There is a name for that technique:
> > cross-compilation. If you want to build DPDK to run on another system with
> > different kernel and libraries, you should adapt the toolchain and libraries
> > headers. And guess what? include/linux/ is part of this toolchain.
> > Problem solved.
> 
> The toolchain Just-works for build igb_uio, kni, etc. Why doesn't it work
> for VFIO. I would view that as a of the mk scripts.

I mean you have to use a cross toolchain including the kernel headers matching
the target kernel. A cross toolchain can be used thanks to the CROSS variable.