On Mon, Dec 23, 2019 at 11:58 AM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 12/20/19 10:26 PM, Martin KaFai Lau wrote:
> > The patch introduces BPF_MAP_TYPE_STRUCT_OPS. The map value
> > is a kernel struct with its func ptr implemented in bpf prog.
> > This new map is the interface to register/unregister/introspect
> > a bpf implemented kernel struct.
> >
> > The kernel struct is actually embedded inside another new struct
> > (or called the "value" struct in the code). For example,
> > "struct tcp_congestion_ops" is embbeded in:
> > struct bpf_struct_ops_tcp_congestion_ops {
> > refcount_t refcnt;
> > enum bpf_struct_ops_state state;
> > struct tcp_congestion_ops data; /* <-- kernel subsystem struct here */
> > }
> > The map value is "struct bpf_struct_ops_tcp_congestion_ops".
> > The "bpftool map dump" will then be able to show the
> > state ("inuse"/"tobefree") and the number of subsystem's refcnt (e.g.
> > number of tcp_sock in the tcp_congestion_ops case). This "value" struct
> > is created automatically by a macro. Having a separate "value" struct
> > will also make extending "struct bpf_struct_ops_XYZ" easier (e.g. adding
> > "void (*init)(void)" to "struct bpf_struct_ops_XYZ" to do some
> > initialization works before registering the struct_ops to the kernel
> > subsystem). The libbpf will take care of finding and populating the
> > "struct bpf_struct_ops_XYZ" from "struct XYZ".
> >
> > Register a struct_ops to a kernel subsystem:
> > 1. Load all needed BPF_PROG_TYPE_STRUCT_OPS prog(s)
> > 2. Create a BPF_MAP_TYPE_STRUCT_OPS with attr->btf_vmlinux_value_type_id
> > set to the btf id "struct bpf_struct_ops_tcp_congestion_ops" of the
> > running kernel.
> > Instead of reusing the attr->btf_value_type_id,
> > btf_vmlinux_value_type_id s added such that attr->btf_fd can still be
> > used as the "user" btf which could store other useful sysadmin/debug
> > info that may be introduced in the furture,
> > e.g. creation-date/compiler-details/map-creator...etc.
> > 3. Create a "struct bpf_struct_ops_tcp_congestion_ops" object as described
> > in the running kernel btf. Populate the value of this object.
> > The function ptr should be populated with the prog fds.
> > 4. Call BPF_MAP_UPDATE with the object created in (3) as
> > the map value. The key is always "0".
> >
> > During BPF_MAP_UPDATE, the code that saves the kernel-func-ptr's
> > args as an array of u64 is generated. BPF_MAP_UPDATE also allows
> > the specific struct_ops to do some final checks in "st_ops->init_member()"
> > (e.g. ensure all mandatory func ptrs are implemented).
> > If everything looks good, it will register this kernel struct
> > to the kernel subsystem. The map will not allow further update
> > from this point.
> >
> > Unregister a struct_ops from the kernel subsystem:
> > BPF_MAP_DELETE with key "0".
> >
> > Introspect a struct_ops:
> > BPF_MAP_LOOKUP_ELEM with key "0". The map value returned will
> > have the prog _id_ populated as the func ptr.
> >
> > The map value state (enum bpf_struct_ops_state) will transit from:
> > INIT (map created) =>
> > INUSE (map updated, i.e. reg) =>
> > TOBEFREE (map value deleted, i.e. unreg)
> >
> > The kernel subsystem needs to call bpf_struct_ops_get() and
> > bpf_struct_ops_put() to manage the "refcnt" in the
> > "struct bpf_struct_ops_XYZ". This patch uses a separate refcnt
> > for the purose of tracking the subsystem usage. Another approach
> > is to reuse the map->refcnt and then "show" (i.e. during map_lookup)
> > the subsystem's usage by doing map->refcnt - map->usercnt to filter out
> > the map-fd/pinned-map usage. However, that will also tie down the
> > future semantics of map->refcnt and map->usercnt.
> >
> > The very first subsystem's refcnt (during reg()) holds one
> > count to map->refcnt. When the very last subsystem's refcnt
> > is gone, it will also release the map->refcnt. All bpf_prog will be
> > freed when the map->refcnt reaches 0 (i.e. during map_free()).
> >
> > Here is how the bpftool map command will look like:
> > [root@arch-fb-vm1 bpf]# bpftool map show
> > 6: struct_ops name dctcp flags 0x0
> > key 4B value 256B max_entries 1 memlock 4096B
> > btf_id 6
> > [root@arch-fb-vm1 bpf]# bpftool map dump id 6
> > [{
> > "value": {
> > "refcnt": {
> > "refs": {
> > "counter": 1
> > }
> > },
> > "state": 1,
>
> The bpftool dump with "state" 1 is a little bit cryptic.
> Since this is common for all struct_ops maps, can we
> make it explicit, e.g., as enum values, like INIT/INUSE/TOBEFREE?
This can (and probably should) be done generically in bpftool for any
field of enum type. Not blocking this patch set, though.
>
> > "data": {
> > "list": {
> > "next": 0,
> > "prev": 0
> > },
> > "key": 0,
> > "flags": 2,
> > "init": 24,
> > "release": 0,
> > "ssthresh": 25,
> > "cong_avoid": 30,
> > "set_state": 27,
> > "cwnd_event": 28,
> > "in_ack_event": 26,
> > "undo_cwnd": 29,
> > "pkts_acked": 0,
> > "min_tso_segs": 0,
> > "sndbuf_expand": 0,
> > "cong_control": 0,
> > "get_info": 0,
> > "name": [98,112,102,95,100,99,116,99,112,0,0,0,0,0,0,0
> > ],
Same here, bpftool should be smart enough to figure out that this is a
string, not just an array of bytes.
> > "owner": 0
> > }
> > }
> > }
> > ]
> >
[...]
