On Tue, Apr 09, 2024 at 06:01:08PM +0300, Eduard Zingerman wrote: > On Tue, 2024-04-09 at 07:56 -0700, Alexei Starovoitov wrote: > [...] > > I would actually go with sorted BTF, since it will probably > > make diff-ing of BTFs practical. Will be easier to track changes What kind of diff-ing of BTFs from different kernels are you interested in? in pahole's repository we have btfdiff, that will, given a vmlinux with both DWARF and BTF use pahole to pretty print all types, expanded, and then compare the two outputs, which should produce the same results from BTF and DWARF. Ditto for DWARF from a vmlinux compared to a detached BTF file. And also now we have another regression test script that will produce the output from 'btftool btf dump' for the BTF generated from DWARF in serial mode, and then compare that with the output from 'bpftool btf dump' for reproducible encodings done using -j 1 ... number-of-processors-on-the-machine. All have to match, all types, all BTF ids. We can as well use something like btfdiff to compare the output from 'pahole --expand_types --sort' for two BTFs for two different kernels, to see what are the new types and the changes to types in both. What else do you want to compare? To be able to match we would have to somehow have ranges for each DWARF CU so that when encoding and then deduplicating we would have space in the ID space for new types to fill in while keeping the old types IDs matching the same types in the new vmlinux. While ordering all types we would have to have ID space available from each of the BTF kinds, no? I haven't looked at Eduard's patches, is that what it is done? > > from one kernel version to another. vmlinux.h will become > > a bit more sorted too and normal diff vmlinux_6_1.h vmlinux_6_2.h > > will be possible. > > Or am I misunderstanding the sorting concept? > You understand the concept correctly, here is a sample: > [1] INT '_Bool' size=1 bits_offset=0 nr_bits=8 encoding=BOOL > [2] INT '__int128' size=16 bits_offset=0 nr_bits=128 encoding=SIGNED > [3] INT '__int128 unsigned' size=16 bits_offset=0 nr_bits=128 encoding=(none) > [4] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=(none) > [5] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED > [6] INT 'long int' size=8 bits_offset=0 nr_bits=64 encoding=SIGNED > [7] INT 'long long int' size=8 bits_offset=0 nr_bits=64 encoding=SIGNED The above: so far so good, probably there will not be something that will push what is now BTF id 6 to become 7 in a new vmlinux, but can we say the same for the more dynamic parts, like the list of structs? A struct can vanish, that abstraction not being used anymore in the kernel, so its BTF id will vacate and all of the next struct IDs will "fall down" and gets its IDs decremented, no? If these difficulties are present as I mentioned, then rebuilding from the BTF data with something like the existing 'pahole --expand_types --sort' from the BTF from kernel N to compare with the same output for kernel N + 1 should be enough to see what changed from one kernel to the next one? - Arnaldo > ... > [15085] STRUCT 'arch_elf_state' size=0 vlen=0 > [15086] STRUCT 'arch_vdso_data' size=0 vlen=0 > [15087] STRUCT 'bpf_run_ctx' size=0 vlen=0 > [15088] STRUCT 'dev_archdata' size=0 vlen=0 > [15089] STRUCT 'dyn_arch_ftrace' size=0 vlen=0 > [15090] STRUCT 'fscrypt_dummy_policy' size=0 vlen=0 > ... > > (Sort by kind, than by vlen, than by name because sorting by name is a > bit costly, then by member properties)
