Re: [PATCH bpf-next v4 3/3] bpf, arm64: use bpf_jit_binary_pack_alloc

[Puranjay Mohan]

Hi Everyone,

[+cc Björn]

On Mon, Jun 26, 2023 at 10:58 AM Puranjay Mohan <puranjay12@xxxxxxxxx> wrote:
>
> Use bpf_jit_binary_pack_alloc for memory management of JIT binaries in
> ARM64 BPF JIT. The bpf_jit_binary_pack_alloc creates a pair of RW and RX
> buffers. The JIT writes the program into the RW buffer. When the JIT is
> done, the program is copied to the final RX buffer
> with bpf_jit_binary_pack_finalize.
>
> Implement bpf_arch_text_copy() and bpf_arch_text_invalidate() for ARM64
> JIT as these functions are required by bpf_jit_binary_pack allocator.
>
> Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
> Acked-by: Song Liu <song@xxxxxxxxxx>

[...]

> +int bpf_arch_text_invalidate(void *dst, size_t len)
> +{
> +       __le32 *ptr;
> +       int ret;
> +
> +       for (ptr = dst; len >= sizeof(u32); len -= sizeof(u32)) {
> +               ret = aarch64_insn_patch_text_nosync(ptr++, AARCH64_BREAK_FAULT);
> +               if (ret)
> +                       return ret;
> +       }
> +
> +       return 0;
> +}
> +

While testing the same patch for riscv bpf jit, Björn found that
./test_tag is taking a lot of
time to complete. He found that bpf_arch_text_invalidate() is calling
patch_text_nosync() in RISCV
and aarch64_insn_patch_text_nosync() here in ARM64. Both of the
implementations call these functions
in a loop for each word. The problem is that every call to
patch_text_nosync()/aarch64_insn_patch_text_nosync()
would clean the cache. This will make this
function(bpf_arch_text_invalidate) really slow.

I did some testing using the vmtest.sh script on ARM64 with and
without the patches, here are the results:

With Prog Pack patches applied:
=========================

root@(none):/# time ./root/bpf/test_tag
test_tag: OK (40945 tests)

real    3m2.001s
user    0m1.644s
sys     2m40.132s

Without Prog Pack:
===============

root@(none):/# time ./root/bpf/test_tag
test_tag: OK (40945 tests)

real    0m26.809s
user    0m1.591s
sys     0m24.106s

As you can see the current implementation of
bpf_arch_text_invalidate() is really slow. I need to
implement a new function: aarch64_insn_set_text_nosync() and use it in
bpf_arch_text_invalidate()
rather than calling aarch64_insn_patch_text_nosync() in a loop.

In the longer run, it would be really helpful if we have a standard
text_poke API like x86 in every architecture.

Thanks,
Puranjay