[PATCH RFC] arm64: eBPF JIT compiler

Hi Will,

On Thu, Jul 3, 2014 at 2:14 AM, Will Deacon [off-list ref] wrote:

Hello,

On Wed, Jul 02, 2014 at 06:20:24AM +0100, Zi Shen Lim wrote:

[...]

First off, this is really cool. Thanks for putting in the effort to get this
supported on arm64! I'm happy to run tests on some real hardware if you tell
me how to run them :)

Thanks for offering to test on real hardware :)
I'm running test_bpf per Documentations/networking/filter.txt, also
described by Daniel in his response to you.

One general observation relates to your instruction encoding logic, e.g:

quoted

+/* 5-bit Register Operand */
+#define A64_R(x)       x               /* R0-R30: General purpose */
+#define A64_FP         A64_R(29)       /* Frame pointer */
+#define A64_LR         A64_R(30)       /* Link register */
+#define A64_ZR         31              /* As source register operand */
+#define A64_SP         31              /* As load/store base register */
+
+#define BITSMASK(bits) ((1 << (bits)) - 1)
+
+/* Compare & branch (immediate) */
+static inline u32 A64_COMP_BRANCH_IMM(int sf, int op, int imm19, int Rt)
+{
+       sf &= BITSMASK(1);
+       op &= BITSMASK(1);
+       imm19 &= BITSMASK(19);
+       Rt &= BITSMASK(5);
+       return 0x34000000 | sf << 31 | op << 24 | imm19 << 5 | Rt;
+}
+#define A64_CBZ(sf, Rt, imm19)  A64_COMP_BRANCH_IMM(sf, 0, imm19, Rt)
+#define A64_CBNZ(sf, Rt, imm19) A64_COMP_BRANCH_IMM(sf, 1, imm19, Rt)

We already have some some basic instruction manipulation code in
arch/arm64/kernel/insn.c and arch/arm64/include/asm/insn.h. Would you be
able to move some of this there please (but only the bits that aren't tied
to BPF?

Ah, thanks for pointing that out to me.

The reason I ask, is because we're inevitebly going to need this stuff
for other subsystems (e.g. kprobes, dynamic code patching ("alternatives"))
and I'd like to avoid a proliferation of magic numbers across the codebase.

Yes, I agree in principle, consolidating this stuff in one place sounds good.

Does this sound remotely feasible?

So I looked at insn.c and the only overlap at this point is B/BL codegen.
A whole lot more, e.g. arithmetic, logical, and memory ops, will need
to be shuffled in.

Let me address Alexei's review comments and send out a v2.
After that, I can take a stab at consolidating bpf_jit.h into
insn.{c,h}. Sounds good to you?

Thanks,
z

Cheers,

Will