Re: [Qemu-devel] [RFC] Bring in all the Linux headers we depend on in QEMU

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:
> Sorry this explanation is long winded, but this is a messy situation.
>
> In Linux, there isn't a very consistent policy about userspace kernel 
> header inclusion.  On a typical Linux system, you're likely to find 
> kernel headers in three places.
>
> glibc headers (/usr/include/{linux,asm})
>
> These headers are installed by glibc.  They very often are based on 
> much older kernel versions that the kernel you have in your 
> distribution.  For software that depends on these headers, very often 
> this means that your software detects features being missing that are 
> present on your kernel.  Furthermore, glibc only installs the headers 
> it needs so very often certain headers have dependencies that aren't 
> met.  A classic example is linux/compiler.h and the broken 
> usbdevice_fs.h header that depends on it.  There are still 
> distributions today that QEMU doesn't compile on because of this.
>
> Today, most of QEMU's code depends on these headers.
>
> /lib/modules/$(uname -r)/build
>
> These are the kernel headers that are installed as part of your 
> kernel.  In general, this is a pretty good place to find the headers 
> that are associated with the kernel version you're actually running 
> on.  However, these headers are part of the kernel build tree and are 
> not always guaranteed to be includable from userspace.
>

I thought these were for external modules, not for userspace.

> <random kernel tree>
>
> Developers, in particular, like to point things at their random kernel 
> trees.  In general though, relying on a full kernel source tree being 
> available isn't a good idea.  Kernel headers change dramatically 
> across versions too so it's very likely that we would need to have a 
> lot of #ifdefs dependent on kernel versions, or some of the uglier 
> work arounds we have in usb-linux.c.
>
> I think the best way to avoid #ifdefs and dependencies on 
> broken/incomplete glibc headers is to include all of the Linux headers 
> we need within QEMU.  The attached patch does just this.
>
> I think there's room for discussion about whether we really want to do 
> this.  We could potentially depend on some more common glibc headers 
> (like asm/types.h) while bringing in less reliable headers 
> (if_tun.h/virtio*).  Including them all seems like the most robust 
> solution to me though.
>
> Comments? 

I think we need to use the output of 'make headers-install', which 
removes things like __user and CONFIG_*.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.