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// Copyright 2022 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//go:build amd64 || arm64
// +build amd64 arm64
package xdp
import (
"golang.org/x/sys/unix"
"gvisor.dev/gvisor/pkg/atomicbitops"
)
// The TXQueue is how a process tells the kernel which buffers are available to
// be sent via the NIC.
//
// TXQueue is not thread-safe and requires external synchronization
type TXQueue struct {
// sockfd is the underlying AF_XDP socket.
sockfd uint32
// mem is the mmap'd area shared with the kernel. Many other fields of
// this struct point into mem.
mem []byte
// ring is the actual ring buffer. It is a list of XDP descriptors
// pointing to ready-to-transmit packets.
//
// len(ring) must be a power of 2.
ring []unix.XDPDesc
// mask is used whenever indexing into ring. It is always len(ring)-1.
// It prevents index out of bounds errors while allowing the producer
// and consumer pointers to repeatedly "overflow" and loop back around
// the ring.
mask uint32
// producer points to the shared atomic value that indicates the last
// produced descriptor. Only we update this value.
producer *atomicbitops.Uint32
// consumer points to the shared atomic value that indicates the last
// consumed descriptor. Only the kernel updates this value.
consumer *atomicbitops.Uint32
// flags points to the shared atomic value that holds flags for the
// queue.
flags *atomicbitops.Uint32
// Cached values are used to avoid relatively expensive atomic
// operations. They are used, incremented, and decremented multiple
// times with non-atomic operations, and then "batch-updated" by
// reading or writing atomically to synchronize with the kernel.
// cachedProducer is used to atomically write *producer.
cachedProducer uint32
// cachedConsumer is updated when we atomically read *consumer.
// cachedConsumer is actually len(ring) larger than the real consumer
// value. See free() for details.
cachedConsumer uint32
}
// Reserve reserves descriptors in the queue. If toReserve descriptors cannot
// be reserved, none are reserved.
//
// +checklocks:umem.mu
func (tq *TXQueue) Reserve(umem *UMEM, toReserve uint32) (nReserved, index uint32) {
if umem.nFreeFrames < toReserve || tq.free(toReserve) < toReserve {
return 0, 0
}
idx := tq.cachedProducer
tq.cachedProducer += toReserve
return toReserve, idx
}
// free returns the number of free descriptors in the TX queue.
func (tq *TXQueue) free(toReserve uint32) uint32 {
// Try to find free descriptors without incurring an atomic operation.
//
// cachedConsumer is always len(tq.ring) larger than the real consumer
// value. This lets us, in the common case, compute the number of free
// descriptors simply via tq.cachedConsumer - tq.cachedProducer without
// also addign len(tq.ring).
if available := tq.cachedConsumer - tq.cachedProducer; available >= toReserve {
return available
}
// If we didn't already have enough descriptors available, check
// whether the kernel has returned some to us.
tq.cachedConsumer = tq.consumer.Load()
tq.cachedConsumer += uint32(len(tq.ring))
return tq.cachedConsumer - tq.cachedProducer
}
// Notify updates the producer such that it is visible to the kernel.
func (tq *TXQueue) Notify() {
tq.producer.Store(tq.cachedProducer)
tq.kick()
}
// Set sets the TX queue's descriptor at index to addr.
func (tq *TXQueue) Set(index uint32, desc unix.XDPDesc) {
// Use mask to avoid overflowing and loop back around the ring.
tq.ring[index&tq.mask] = desc
}
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