On Mon, Jan 24, 2022 at 11:21 PM Song Liu <song@xxxxxxxxxx> wrote: > > On Mon, Jan 24, 2022 at 9:21 PM Alexei Starovoitov > <alexei.starovoitov@xxxxxxxxx> wrote: > > > > On Mon, Jan 24, 2022 at 10:27 AM Song Liu <songliubraving@xxxxxx> wrote: > > > > > > > > Are arches expected to allocate rw buffers in different ways? If not, > > > > I would consider putting this into the common code as well. Then > > > > arch-specific code would do something like > > > > > > > > header = bpf_jit_binary_alloc_pack(size, &prg_buf, &prg_addr, ...); > > > > ... > > > > /* > > > > * Generate code into prg_buf, the code should assume that its first > > > > * byte is located at prg_addr. > > > > */ > > > > ... > > > > bpf_jit_binary_finalize_pack(header, prg_buf); > > > > > > > > where bpf_jit_binary_finalize_pack() would copy prg_buf to header and > > > > free it. > > > > It feels right, but bpf_jit_binary_finalize_pack() sounds 100% arch > > dependent. The only thing it will do is perform a copy via text_poke. > > What else? > > > > > I think this should work. > > > > > > We will need an API like: bpf_arch_text_copy, which uses text_poke_copy() > > > for x86_64 and s390_kernel_write() for x390. We will use bpf_arch_text_copy > > > to > > > 1) write header->size; > > > 2) do finally copy in bpf_jit_binary_finalize_pack(). > > > > we can combine all text_poke operations into one. > > > > Can we add an 'image' pointer into struct bpf_binary_header ? > > There is a 4-byte hole in bpf_binary_header. How about we put > image_offset there? Actually we only need 2 bytes for offset. > > > Then do: > > int bpf_jit_binary_alloc_pack(size, &ro_hdr, &rw_hdr); > > > > ro_hdr->image would be the address used to compute offsets by JIT. > > If we only do one text_poke(), we cannot write ro_hdr->image yet. We > can use ro_hdr + rw_hdr->image_offset instead. Good points. Maybe let's go back to Ilya's suggestion and return 4 pointers from bpf_jit_binary_alloc_pack ? > > rw_hdr->image would point to kvmalloc-ed area for emitting insns. > > rw_hdr->size would already be populated. > > > > The JITs would write insns into rw_hdr->image including 'int 3' insns. > > At the end the JIT will do text_poke_copy(ro_hdr, rw_hdr, rw_hdr->size); > > That would be the only copy that will transfer everything into final > > location. > > Then kvfree(rw_hdr) > > The only problem is the asymmetry of allocating rw_hdr from bpf/core.c, > and freeing it from arch/bpf_jit_comp.c. But it doesn't bother me too much. Indeed. Asymmetry needs to be fixed. Let's then pass 4 pointers back into bpf_jit_binary_finalize_pack() which will call arch dependent weak function to do text_poke_copy or use default __weak function that returns eopnotsupp and then kvfree the rw_hdr ? I'd like to avoid callbacks. imo __weak is easier to follow.
