On Wed, Oct 14, 2020 at 12:05 AM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > On Fri, Oct 9, 2020 at 12:58 PM Juraj Vijtiuk <juraj.vijtiuk@xxxxxxxxxx> wrote: > > > > It would be great to hear if anyone has any thoughts on running a set > > of BPF programs JITed while other programs are run by the interpreter. > > > > Something like that would be useful on 32-bit architectures, as the > > JIT compiler there doesn't support some instructions, primarily > > instructions that work with 64-bit data. As far as I can tell, it is > > unlikely that support will be coming soon as it is a general issue for > > all 32-bit architectures. Atomic operations like BPF_XADD look > > especially problematic regarding support on 32 bit platforms. From > > what I managed to see such a conclusion appeared in a few patches > > where support for 32-bit JITs was added, for example [0]. > > That results in some programs being runnable with BPF JIT enabled, and > > some failing during load time, but running successfully without JIT on > > 32-bit platforms. > > > > The only way to run some programs with JIT and some without, that > > seems possible right now, is to manually change > > /proc/sys/net/core/bpf_jit_enable every time a program is loaded. > > Although I've managed to do that and it seems to be working, it seems > > pretty hacky and looks like it could cause race conditions if multiple > > programs were loaded, especially by independent loaders. > > I agree, the global file is not flexible enough and can cause problems > in production environment. > > I don't see any reason why we shouldn't allow to decide interpreted vs > jitted mode per program during BPF_PROG_LOAD. > > See kernel/bpf/core.c, bpf_prog's jit_requested field determines > whether a program is going to be jitted or not. It should be trivial > to allow overriding that during BPF_PROG_LOAD command. > > We can probably also generalize this to allow to "force-jit" or > "force-interpret" by users, which would fail if kernel didn't support > requested mode. > Thanks for the suggestion, that makes sense. I've started working on a patch today. I'll post again when I get something working and test it. > > > > At first glance it seems that if something like this was to be added > > to a loader, it would have to either somehow be aware of other BPF > > programs being loaded or possibly implement some sort of locking > > mechanism which also seems hacky. From what I understand, doing it in > > the kernel looks even less promising as bpf_jit_enable is a system > > wide setting, and I imagine that changing it to work on a per program > > basis would pretty much require a rework of the current design, so > > that looks even less promising. > > > > It looks like the best option right now is to just run everything in > > interpreted mode, but I want to make sure that I am not missing > > something. If someone has tried doing something similar, it would be > > great to know about that. > > > > Thanks, > > Juraj Vijtiuk > > > > [0] https://lore.kernel.org/netdev/20200305050207.4159-3-luke.r.nels@xxxxxxxxx/
