Copilot commented on code in PR #3304:
URL: https://github.com/apache/brpc/pull/3304#discussion_r3709071177
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src/brpc/rdma/rdma_endpoint.cpp:
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@@ -1188,7 +1188,24 @@ int RdmaEndpoint::BringUpQp(const ParsedHello& remote,
bool is_server) {
}
attr.qp_state = IBV_QPS_RTR;
- attr.path_mtu = IBV_MTU_1024; // TODO: support more mtu in future
+ // MTU negotiation: use the peer-advertised MTU if available, otherwise
+ // fall back to the legacy default (IBV_MTU_1024).
+ // Server side: remote.path_mtu is the client's active MTU;
+ // we compute min(local, client) here.
+ // Client side: remote.path_mtu is already the server's negotiated
+ // min(local, client) and we use it as-is.
+ uint32_t negotiated_mtu = IBV_MTU_1024;
+ if (remote.path_mtu.has_value()) {
+ if (is_server) {
+ uint32_t local_mtu = GetRdmaActiveMtu();
+ negotiated_mtu = std::min(local_mtu, *remote.path_mtu);
+ // Store the negotiated MTU for the server hello reply.
+ _outgoing_mtu = negotiated_mtu;
+ } else {
+ negotiated_mtu = *remote.path_mtu;
+ }
+ }
+ attr.path_mtu = static_cast<ibv_mtu>(negotiated_mtu);
Review Comment:
remote.path_mtu comes from the peer and is used directly to configure
attr.path_mtu via static_cast<ibv_mtu>(negotiated_mtu). If a peer sends an
out-of-range value (including 0), this can select an invalid ibv_mtu and make
IbvModifyQp fail (handshake DoS). Validate the MTU is one of
IBV_MTU_{256,512,1024,2048,4096} before using it; otherwise ignore it and fall
back to IBV_MTU_1024.
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