kernel features MIB

[John Baldwin]

One of the things we have at work is a kern.features sysctl MIB that contains 
nodes to indicate if a named feature is present.  For example, on i386 we 
have kern.features.pae and we auto enable -DPAE for kernel modules if the 
currently running kernel is using PAE using that sysctl.

One of the patches I want to commit soon is support for handling 
shm_open/shm_unlink directly in the kernel via swap-backed VM objects (the 
long-heralded memfd stuff).  I would like to have the sysctl MIB so that 
libc's for older releases (e.g. libc.so.6) could use the syscalls if they are 
available so that shm segments are shared between compat apps (e.g. 4.x or 
6.x) and up-to-date apps.

At work we don't have a pretty API for this at all, but I'm thinking for 
FreeBSD we can do this:

FEATURE(foo, "description of foo")

which is a macro to create the 'kern.features.foo' node and set it to 1.  Then 
we could have a routine in libc:

int	feature_present(const char *name);

That returns a boolean to indicate if a given feature is present or not by 
invoking sysctlbyname(3), etc.

Any objections to the idea?

-- 
John Baldwin