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?
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.
