On Wed, May 21, 2025 at 8:00 AM Alan Maguire <alan.magu...@oracle.com> wrote: > > > Hi Alan, > > > > Thanks for taking a look at this. I've been following your related effort > > to allow /sys/kernel/btf/vmlinux as a module in support of small systems > > with kernel-size constraints, and wondered how this series might affect > > that work? Such support would be well-received in the embedded space when > > it happens, so am keen to understand. > > > > Thanks, > > Tony > > hi Tony > > I had something nearly working a few months back but there are a bunch > of complications that made it a bit trickier than I'd first anticipated. > One challenge for example is that we want /sys/kernel/btf to behave just > as it would if vmlinux BTF was not a module. My original hope was to > just have the vmlinux BTF module forceload early, but the request module > approach won't work since the vmlinux_btf.ko module would have to be > part of the initrd image. A question for you on this - I presume that's > what you want to avoid, right? So I'm assuming that we need to extract > the .BTF section out of the vmlinu[xz] binary and out of initrd into a > later-loading vmlinux_btf.ko module for small-footprint systems. Is that > correct? > > The reason I ask is having a later-loading vmlinux_btf.ko is a bit of a > pain since we need to walk the set of kernel modules and load their BTF, > relocate it and do kfunc registration. If we can simplify things via a > shared module dependency on vmlinux_btf.ko that would be great, but I'd > like to better understand the constraints from the small system > perspective first. Thanks!
We cannot require other modules to depend on vmlinux_btf.ko. Some of them might load during the boot. So adding to the dependency will defeat the point of vmlinux_btf.ko. The only option I see is to let modules load and ignore their BTFs and vmlinux BTF is not present. Later vmlinux_btf.ko can be loaded and modules loaded after that time will succeed in loading their BTFs too. So some modules will have their BTF and some don't. I don't think it's an issue. If an admin loads a module with kfuncs and vmlixnu_btf.ko is not loaded yet the kfunc registration will fail, of course. It's an issue, but I don't think we need to fix it right now by messing with depmod. The bigger issue is how to split vmlinux_btf.ko itself. The kernel has a bunch of kfuncs and they need BTF ids for protos and for all types they reference, so vmlinux BTF cannot be empty. minimize_btf() can probably help. So before we proceed with vmlinux_btf.ko we need to see the data how big the mandatory part of vmlinux BTF will be vs the rest of BTF in vmlinux_btf.ko.
