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The BPF dispatcher is a multi-way branch code generator, mainly targeted for XDP programs. When an XDP program is executed via the bpf_prog_run_xdp(), it is invoked via an indirect call. The indirect call has a substantial performance impact, when retpolines are enabled. The dispatcher transform indirect calls to direct calls, and therefore avoids the retpoline. The dispatcher is generated using the BPF JIT, and relies on text poking provided by bpf_arch_text_poke(). The dispatcher hijacks a trampoline function it via the __fentry__ nop of the trampoline. One dispatcher instance currently supports up to 64 dispatch points. A user creates a dispatcher with its corresponding trampoline with the DEFINE_BPF_DISPATCHER macro. Signed-off-by: Björn Töpel <bjorn.topel@intel.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/20191213175112.30208-3-bjorn.topel@gmail.com
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Björn Töpel
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Alexei Starovoitov
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Dec 13, 2019
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| Original file line number | Diff line number | Diff line change |
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| // SPDX-License-Identifier: GPL-2.0-only | ||
| /* Copyright(c) 2019 Intel Corporation. */ | ||
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| #include <linux/hash.h> | ||
| #include <linux/bpf.h> | ||
| #include <linux/filter.h> | ||
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| /* The BPF dispatcher is a multiway branch code generator. The | ||
| * dispatcher is a mechanism to avoid the performance penalty of an | ||
| * indirect call, which is expensive when retpolines are enabled. A | ||
| * dispatch client registers a BPF program into the dispatcher, and if | ||
| * there is available room in the dispatcher a direct call to the BPF | ||
| * program will be generated. All calls to the BPF programs called via | ||
| * the dispatcher will then be a direct call, instead of an | ||
| * indirect. The dispatcher hijacks a trampoline function it via the | ||
| * __fentry__ of the trampoline. The trampoline function has the | ||
| * following signature: | ||
| * | ||
| * unsigned int trampoline(const void *ctx, const struct bpf_insn *insnsi, | ||
| * unsigned int (*bpf_func)(const void *, | ||
| * const struct bpf_insn *)); | ||
| */ | ||
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| static struct bpf_dispatcher_prog *bpf_dispatcher_find_prog( | ||
| struct bpf_dispatcher *d, struct bpf_prog *prog) | ||
| { | ||
| int i; | ||
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| for (i = 0; i < BPF_DISPATCHER_MAX; i++) { | ||
| if (prog == d->progs[i].prog) | ||
| return &d->progs[i]; | ||
| } | ||
| return NULL; | ||
| } | ||
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| static struct bpf_dispatcher_prog *bpf_dispatcher_find_free( | ||
| struct bpf_dispatcher *d) | ||
| { | ||
| return bpf_dispatcher_find_prog(d, NULL); | ||
| } | ||
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| static bool bpf_dispatcher_add_prog(struct bpf_dispatcher *d, | ||
| struct bpf_prog *prog) | ||
| { | ||
| struct bpf_dispatcher_prog *entry; | ||
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| if (!prog) | ||
| return false; | ||
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| entry = bpf_dispatcher_find_prog(d, prog); | ||
| if (entry) { | ||
| refcount_inc(&entry->users); | ||
| return false; | ||
| } | ||
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| entry = bpf_dispatcher_find_free(d); | ||
| if (!entry) | ||
| return false; | ||
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| bpf_prog_inc(prog); | ||
| entry->prog = prog; | ||
| refcount_set(&entry->users, 1); | ||
| d->num_progs++; | ||
| return true; | ||
| } | ||
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| static bool bpf_dispatcher_remove_prog(struct bpf_dispatcher *d, | ||
| struct bpf_prog *prog) | ||
| { | ||
| struct bpf_dispatcher_prog *entry; | ||
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| if (!prog) | ||
| return false; | ||
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| entry = bpf_dispatcher_find_prog(d, prog); | ||
| if (!entry) | ||
| return false; | ||
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| if (refcount_dec_and_test(&entry->users)) { | ||
| entry->prog = NULL; | ||
| bpf_prog_put(prog); | ||
| d->num_progs--; | ||
| return true; | ||
| } | ||
| return false; | ||
| } | ||
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| int __weak arch_prepare_bpf_dispatcher(void *image, s64 *funcs, int num_funcs) | ||
| { | ||
| return -ENOTSUPP; | ||
| } | ||
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| static int bpf_dispatcher_prepare(struct bpf_dispatcher *d, void *image) | ||
| { | ||
| s64 ips[BPF_DISPATCHER_MAX] = {}, *ipsp = &ips[0]; | ||
| int i; | ||
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| for (i = 0; i < BPF_DISPATCHER_MAX; i++) { | ||
| if (d->progs[i].prog) | ||
| *ipsp++ = (s64)(uintptr_t)d->progs[i].prog->bpf_func; | ||
| } | ||
| return arch_prepare_bpf_dispatcher(image, &ips[0], d->num_progs); | ||
| } | ||
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| static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs) | ||
| { | ||
| void *old, *new; | ||
| u32 noff; | ||
| int err; | ||
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| if (!prev_num_progs) { | ||
| old = NULL; | ||
| noff = 0; | ||
| } else { | ||
| old = d->image + d->image_off; | ||
| noff = d->image_off ^ (PAGE_SIZE / 2); | ||
| } | ||
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| new = d->num_progs ? d->image + noff : NULL; | ||
| if (new) { | ||
| if (bpf_dispatcher_prepare(d, new)) | ||
| return; | ||
| } | ||
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| err = bpf_arch_text_poke(d->func, BPF_MOD_JUMP, old, new); | ||
| if (err || !new) | ||
| return; | ||
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| d->image_off = noff; | ||
| } | ||
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| void bpf_dispatcher_change_prog(struct bpf_dispatcher *d, struct bpf_prog *from, | ||
| struct bpf_prog *to) | ||
| { | ||
| bool changed = false; | ||
| int prev_num_progs; | ||
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| if (from == to) | ||
| return; | ||
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| mutex_lock(&d->mutex); | ||
| if (!d->image) { | ||
| d->image = bpf_jit_alloc_exec_page(); | ||
| if (!d->image) | ||
| goto out; | ||
| } | ||
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| prev_num_progs = d->num_progs; | ||
| changed |= bpf_dispatcher_remove_prog(d, from); | ||
| changed |= bpf_dispatcher_add_prog(d, to); | ||
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| if (!changed) | ||
| goto out; | ||
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| bpf_dispatcher_update(d, prev_num_progs); | ||
| out: | ||
| mutex_unlock(&d->mutex); | ||
| } |