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.
<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?
Thinking again about it, this is not really necessary.
In general a distro provides kernel headers matched to the running
kernel. For example F10 provides
kernel-headers-2.6.27.21-170.2.56.fc10.x86_64 to go along with
kernel-2.6.27.21-170.2.56.fc10.x86_64. So a user running a distro
kernel (the majority, given that most people don't inflict pain upon
themselves unnecessarily) will have exactly the features exported by the
kernel.
If a user compiles their own kernel, they will also have the complete
kernel sources. We could use --kerneldir, perhaps requiring that the
user do a 'make headers-install' first and point kerneldir to the result.
The only deviation for this is kvm, which also comes as an external
kernel module and therefore cannot rely on the installed kernel
headers. We could make the external module package (kvm-kmod) supply
its own set of headers and install them somewhere, or we can carry them
in qemu (much more convenient). But I don't think we need to carry such
a large subset of the kernel headers (which is liable to change as
kernel headers are added).
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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