Linux Virtualization - May 2013 - [Qemu-devel] [PATCH v2 00/11] qemu: use virtio linux headers in portable code

Paolo Bonzini <pbonzini at redhat.com> writes:
> Il 26/05/2013 22:02, Michael S. Tsirkin ha scritto:
>> > My fault.  I should have looked at linux/types.h (actually
asm-generic/).
>> 
>> Not really, __uX appear in the headers that were posted.
Which is a problem because this is a reserved namespace in C99.
>> What I'm saying is - a chance of a conflict is very remote,
>> if it happens it's a build failure so easy to debug.
>
> I'm sure that others will complain, :) but you can go ahead.
I think we should clean up the virtio headers in the kernel first.
Seems like a good thing to do given the standardization effort too.

There are lots of headers in uapi that use the C99 int types so there
doesn't seem to be a reason they couldn't be used in virtio.  fuse.h
even has a:

    #ifdef __KERNEL__
    #include <linux/types.h>
    #else
    #include <stdint.h>
    #endif

Which seems like a reasonable thing to do.

The only other kernel dependency is linux/if_ether.h to define
ETH_ALEN.  But it seems completely reasonable to define a
VIRTIO_NET_MAC_ALEN or something like that.

This would make the virtio headers completely stand alone and includable
in userspace (without violating C99).

Perhaps it's even worth moving the headers from uapi/linux to
uapi/virtio.  Rusty, what do you think?

Regards,

Anhtony Liguori
>
> Paolo

On Sun, May 26, 2013 at 03:49:53PM -0500, Anthony Liguori
wrote:
> Paolo Bonzini <pbonzini at redhat.com> writes:
> 
> > Il 26/05/2013 22:02, Michael S. Tsirkin ha scritto:
> >> > My fault.  I should have looked at linux/types.h (actually
asm-generic/).
> >> 
> >> Not really, __uX appear in the headers that were posted.
> 
> Which is a problem because this is a reserved namespace in C99.
> 
> >> What I'm saying is - a chance of a conflict is very remote,
> >> if it happens it's a build failure so easy to debug.
> >
> > I'm sure that others will complain, :) but you can go ahead.
> 
> I think we should clean up the virtio headers in the kernel first.
> Seems like a good thing to do given the standardization effort too.
> There are lots of headers in uapi that use the C99 int types
I found 4:
$ grep -l uint include/uapi/linux/*h
include/uapi/linux/dm-log-userspace.h
include/uapi/linux/fuse.h
include/uapi/linux/jffs2.h
include/uapi/linux/pps.h
include/uapi/linux/rds.h
include/uapi/linux/sctp.h

That's not really lots.
> so there
> doesn't seem to be a reason they couldn't be used in virtio. 
fuse.h
> even has a:
> 
>     #ifdef __KERNEL__
>     #include <linux/types.h>
>     #else
>     #include <stdint.h>
>     #endif
> 
> Which seems like a reasonable thing to do.
In kernel, we really want to use things like endian-ness
checks (and, we really should have them in userspace too!).
So we want __le32 and other kernel goodies
like that. stdint.h won't cut it.
> The only other kernel dependency is linux/if_ether.h to define
> ETH_ALEN.  But it seems completely reasonable to define a
> VIRTIO_NET_MAC_ALEN or something like that.
Ugh. Not really reasonable IMO. We also use ETH_P_IP in code,
would like to get rid of redefining that too.
But we can have our own linux/if_ether.h for non-linux hosts,
just with a
couple of macros like that, it's not a big deal.
> This would make the virtio headers completely stand alone and includable
> in userspace (without violating C99).
> 
> Perhaps it's even worth moving the headers from uapi/linux to
> uapi/virtio.  Rusty, what do you think?
> 
> Regards,
> 
> Anhtony Liguori
> 
> >
> > Paolo

Anthony Liguori <anthony at codemonkey.ws> writes:
> Paolo Bonzini <pbonzini at redhat.com> writes:
>
>> Il 26/05/2013 22:02, Michael S. Tsirkin ha scritto:
>>> > My fault.  I should have looked at linux/types.h (actually
asm-generic/).
>>> 
>>> Not really, __uX appear in the headers that were posted.
>
> Which is a problem because this is a reserved namespace in C99.
Personally, I find it hard to care.  What matters is not what the
standard has carved out, but whether we have clashes, reserved namespace
or no.  And that won't happen for these.

If someone wants to convert all the kernel headers, I won't NAK it
though.
> Perhaps it's even worth moving the headers from uapi/linux to
> uapi/virtio.  Rusty, what do you think?
Hmm, #include <virtio/virtio_net.h> etc would be worthwhile if that also
worked on FreeBSD.  Bryan CC'd...

Cheers,
Rusty.