powerpc64/bpf: Add support for bpf arena and arena atomics #5862
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September 4, 2025 01:18
Add support for [LDX | STX | ST], PROBE_MEM32, [B | H | W | DW] instructions. They are similar to PROBE_MEM instructions with the following differences: - PROBE_MEM32 supports store. - PROBE_MEM32 relies on the verifier to clear upper 32-bit of the src/dst register - PROBE_MEM32 adds 64-bit kern_vm_start address (which is stored in _R26 in the prologue). Due to bpf_arena constructions such _R26 + reg + off16 access is guaranteed to be within arena virtual range, so no address check at run-time. - PROBE_MEM32 allows STX and ST. If they fault the store is a nop. When LDX faults the destination register is zeroed. To support these on powerpc, we do tmp1 = _R26 + src/dst reg and then use tmp1 as the new src/dst register. This allows us to reuse most of the code for normal [LDX | STX | ST]. Additionally, bpf_jit_emit_probe_mem_store() is introduced to emit instructions for storing memory values depending on the size (byte, halfword, word, doubleword). Stack layout is adjusted to introduce a new NVR (_R26) and to make BPF_PPC_STACKFRAME quadword aligned (local_tmp_var is increased by 8 bytes). Signed-off-by: Saket Kumar Bhaskar <[email protected]> Reviewed-by: Hari Bathini <[email protected]>
LLVM generates bpf_addr_space_cast instruction while translating pointers between native (zero) address space and __attribute__((address_space(N))). The addr_space=0 is reserved as bpf_arena address space. rY = addr_space_cast(rX, 0, 1) is processed by the verifier and converted to normal 32-bit move: wX = wY. rY = addr_space_cast(rX, 1, 0) : used to convert a bpf arena pointer to a pointer in the userspace vma. This has to be converted by the JIT. PPC_RAW_RLDICL_DOT, a variant of PPC_RAW_RLDICL is introduced to set condition register as well. Signed-off-by: Saket Kumar Bhaskar <[email protected]>
…tructions The existing code for emitting bpf atomic instruction sequences for atomic operations such as XCHG, CMPXCHG, ADD, AND, OR, and XOR has been refactored into a reusable function, bpf_jit_emit_ppc_atomic_op(). It also computes the jump offset and tracks the instruction index for jited LDARX/LWARX to be used in case it causes a fault. Signed-off-by: Saket Kumar Bhaskar <[email protected]>
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September 4, 2025 01:18
powerpc supports BPF atomic operations using a loop around Load-And-Reserve(LDARX/LWARX) and Store-Conditional(STDCX/STWCX) instructions gated by sync instructions to enforce full ordering. To implement arena_atomics, arena vm start address is added to the dst_reg to be used for both the LDARX/LWARX and STDCX/STWCX instructions. Further, an exception table entry is added for LDARX/LWARX instruction to land after the loop on fault. At the end of sequence, dst_reg is restored by subtracting arena vm start address. bpf_jit_supports_insn() is introduced to selectively enable instruction support as in other architectures like x86 and arm64. Signed-off-by: Saket Kumar Bhaskar <[email protected]>
For systems having CONFIG_NR_CPUS set to > 1024 in kernel config the selftest fails as arena_spin_lock_irqsave() returns EOPNOTSUPP. The selftest is skipped incase bpf program returns EOPNOTSUPP, with a descriptive message logged. Signed-off-by: Saket Kumar Bhaskar <[email protected]>
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Pull request for series with
subject: powerpc64/bpf: Add support for bpf arena and arena atomics
version: 2
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=996997