Hi Alex, On Tue, 22 Jun 2021 09:31:33 +0200 Alex Ghiti <alex@xxxxxxxx> wrote: > Hi Alexei, > > Le 22/06/2021 à 02:28, Alexei Starovoitov a écrit : > > On Sun, Jun 20, 2021 at 11:43 PM Alex Ghiti <alex@xxxxxxxx> wrote: > >> > >> Hi, > >> > >> Le 18/06/2021 à 19:32, Andrii Nakryiko a écrit : > >>> On Fri, Jun 18, 2021 at 2:13 AM Alex Ghiti <alex@xxxxxxxx> wrote: > >>>> > >>>> Hi guys, > >>>> > >>>> First, pardon my ignorance regarding BPF, the following might be silly. > >>>> > >>>> We were wondering here > >>>> https://patchwork.kernel.org/project/linux-riscv/patch/20210615004928.2d27d2ac@xhacker/ > >>>> if BPF programs that now have the capability to call kernel functions > >>>> (https://lwn.net/Articles/856005/) can also call modules function or > >>>> vice-versa? > >>> > >>> Not yet, but it was an explicit design consideration and there was > >>> public interest just recently. So I'd say this is going to happen > >>> sooner rather than later. > >>> > >>>> > >>>> The underlying important fact is that in riscv, we are limited to 2GB > >>>> offset to call functions and that restricts where we can place modules > >>>> and BPF regions wrt kernel (see Documentation/riscv/vm-layout.rst for > >>>> the current possibly wrong layout). > >>>> > >>>> So should we make sure that modules and BPF lie in the same 2GB region? > >>> > >>> Based on the above and what you are explaining about 2GB limits, I'd > >>> say yes?.. Or alternatively those 2GB restrictions might perhaps be > >>> lifted somehow? > >> > >> > >> Actually we have this limit when we have PC-relative branch which is our > >> current code model. To better understand what happened, I took a look at > >> our JIT implementation and noticed that BPF_CALL are implemented using > >> absolute addressing so for this pseudo-instruction, the limit I evoked > >> does not apply. How are the kernel (and modules) symbol addresses > >> resolved? Is it relative or absolute? Is there then any guarantee that a > >> kernel or module call will always emit a BPF_CALL? > > > > Are those questions for riscv bpf JIT experts? > > Yes more or less, sorry about that, I added Bjorn in cc in case he wants > to intervene. But I think my last question is relevant: Is there then > any guarantee that a kernel or module call will always emit a BPF_CALL? Just my humble opinion: not always, but some kernel/module functions may call BPF. The problem is we dunno where the kernel or module functions sit. > Because that would mean that we don't need to place BPF close to modules > since BPF_CALL are JITed into an absolute branch in riscv. > > Sorry to bother, > > Thanks you for your time, > > Alex > > > Like 'relative or absolute' depends on arch. > > On x86-64 BPF_CALL is JITed into single x86 call instruction that > > has 32-bit immediate which is PC relative. > > Every JIT picks what's the best for that particular arch. > >
