On Sat, 9 Nov 2019 00:06:30 -0800, Andrii Nakryiko wrote: > With BPF_F_MMAPABLE array allocating data in separate chunk of memory, > array_map_gen_lookup has to accomodate these changes. For non-memory-mapped > there are no changes and no extra instructions. For BPF_F_MMAPABLE case, > pointer to where array data is stored has to be dereferenced first. > > Generated code for non-memory-mapped array: > > ; p = bpf_map_lookup_elem(&data_map, &zero); > 22: (18) r1 = map[id:19] > 24: (07) r1 += 408 /* array->inline_data offset */ > 25: (61) r0 = *(u32 *)(r2 +0) > 26: (35) if r0 >= 0x3 goto pc+3 > 27: (67) r0 <<= 3 > 28: (0f) r0 += r1 > 29: (05) goto pc+1 > 30: (b7) r0 = 0 > > Generated code for memory-mapped array: > > ; p = bpf_map_lookup_elem(&data_map, &zero); > 22: (18) r1 = map[id:27] > 24: (07) r1 += 400 /* array->data offset */ > 25: (79) r1 = *(u64 *)(r1 +0) /* extra dereference */ > 26: (61) r0 = *(u32 *)(r2 +0) > 27: (35) if r0 >= 0x3 goto pc+3 > 28: (67) r0 <<= 3 > 29: (0f) r0 += r1 > 30: (05) goto pc+1 > 31: (b7) r0 = 0 Would it not be possible to overallocate the memory and align the start of the bpf_map in case of BPF_F_MMAPABLE so that no extra dereference is needed?
