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riscv, bpf: add internal-only MOV instruction to resolve per-CPU addrs

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Support an instruction for resolving absolute addresses of per-CPU
data from their per-CPU offsets. This instruction is internal-only and
users are not allowed to use them directly. They will only be used for
internal inlining optimizations for now between BPF verifier and BPF
JITs.

RISC-V uses generic per-cpu implementation where the offsets for CPUs
are kept in an array called __per_cpu_offset[cpu_number]. RISCV stores
the address of the task_struct in TP register. The first element in
task_struct is struct thread_info, and we can get the cpu number by
reading from the TP register + offsetof(struct thread_info, cpu).

Once we have the cpu number in a register we read the offset for that
cpu from address: &__per_cpu_offset + cpu_number << 3. Then we add this
offset to the destination register.

To measure the improvement from this change, the benchmark in [1] was
used on Qemu:

Before:
glob-arr-inc   :    1.127 ± 0.013M/s
arr-inc        :    1.121 ± 0.004M/s
hash-inc       :    0.681 ± 0.052M/s

After:
glob-arr-inc   :    1.138 ± 0.011M/s
arr-inc        :    1.366 ± 0.006M/s
hash-inc       :    0.676 ± 0.001M/s

[1] https://github.com/anakryiko/linux/commit/8dec900975ef

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/r/20240502151854.9810-2-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Puranjay Mohan 2024-05-02 15:18:51 +00:00 committed by Alexei Starovoitov
parent f122668ddc
commit 19c56d4e5b
1 changed files with 24 additions and 0 deletions
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24
arch/riscv/net/bpf_jit_comp64.c
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@ -12,6 +12,7 @@
#include <linux/stop_machine.h>
#include <asm/patch.h>
#include <asm/cfi.h>
#include <asm/percpu.h>
#include "bpf_jit.h"
#define RV_FENTRY_NINSNS 2
@ -1089,6 +1090,24 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
emit_or(RV_REG_T1, rd, RV_REG_T1, ctx);
emit_mv(rd, RV_REG_T1, ctx);
break;
} else if (insn_is_mov_percpu_addr(insn)) {
if (rd != rs)
emit_mv(rd, rs, ctx);
#ifdef CONFIG_SMP
/* Load current CPU number in T1 */
emit_ld(RV_REG_T1, offsetof(struct thread_info, cpu),
RV_REG_TP, ctx);
/* << 3 because offsets are 8 bytes */
emit_slli(RV_REG_T1, RV_REG_T1, 3, ctx);
/* Load address of __per_cpu_offset array in T2 */
emit_addr(RV_REG_T2, (u64)&__per_cpu_offset, extra_pass, ctx);
/* Add offset of current CPU to __per_cpu_offset */
emit_add(RV_REG_T1, RV_REG_T2, RV_REG_T1, ctx);
/* Load __per_cpu_offset[cpu] in T1 */
emit_ld(RV_REG_T1, 0, RV_REG_T1, ctx);
/* Add the offset to Rd */
emit_add(rd, rd, RV_REG_T1, ctx);
#endif
}
if (imm == 1) {
/* Special mov32 for zext */
@ -2038,3 +2057,8 @@ bool bpf_jit_supports_arena(void)
{
return true;
}
bool bpf_jit_supports_percpu_insn(void)
{
return true;
}