From 8dff32eea519c77d5d13f1aaa66d52bb62514a17 Mon Sep 17 00:00:00 2001
From: Avi Kivity <avi@cloudius-systems.com>
Date: Fri, 7 Nov 2014 13:09:24 +0200
Subject: [PATCH] xen: simplify free grant table ref id management

There is no reason to wait when pushing back a free id - there is nothing
that could possibly block there.

Switch from a queue<> to an std::queue<> and use a semaphore to guard
popping from the queue.
---
 net/xenfront.cc | 18 +++++++++++-------
 net/xenfront.hh |  7 ++++---
 2 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/net/xenfront.cc b/net/xenfront.cc
index a74cede9e3..ecb153ca4e 100644
--- a/net/xenfront.cc
+++ b/net/xenfront.cc
@@ -158,12 +158,17 @@ xenfront_net_device::send(packet _p) {
 
 template <typename T>
 future<unsigned> front_ring<T>::entries::get_index() {
-    return _ids.pop_eventually();
+    return _available.wait().then([this] {
+        auto ret = _ids.front();
+        _ids.pop();
+        return make_ready_future<unsigned>(ret);
+    });
 }
 
 template <typename T>
-future<> front_ring<T>::entries::free_index(unsigned id) {
-    return _ids.push_eventually(std::move(id));
+void front_ring<T>::entries::free_index(unsigned id) {
+    _ids.push(id);
+    _available.signal();
 }
 
 future<> xenfront_net_device::queue_rx_packet() {
@@ -198,9 +203,8 @@ future<> xenfront_net_device::queue_rx_packet() {
         rsp_prod = _rx_ring._sring->rsp_prod;
 
         _rx_refs->free_ref(entry);
-        _rx_ring.entries.free_index(rsp.id).then([this, id = rsp.id]() {
-            alloc_one_rx_reference(id);
-        });
+        _rx_ring.entries.free_index(rsp.id);
+        alloc_one_rx_reference(rsp.id);
     }
 
     // FIXME: Queue_rx maybe should not be a future then
@@ -252,7 +256,7 @@ future<> xenfront_net_device::handle_tx_completions() {
 
         auto entry = _tx_ring.entries[rsp.id];
         _tx_refs->free_ref(entry);
-        _tx_ring.entries.free_index(rsp.id).then([this]() {});
+        _tx_ring.entries.free_index(rsp.id);
     }
     _tx_ring.rsp_cons = prod;
     return make_ready_future<>();
diff --git a/net/xenfront.hh b/net/xenfront.hh
index 04ebb1681c..f8440ec465 100644
--- a/net/xenfront.hh
+++ b/net/xenfront.hh
@@ -73,12 +73,13 @@ class front_ring {
 public:
     class entries {
     protected:
-        queue<unsigned> _ids;
+        std::queue<unsigned, circular_buffer<unsigned>> _ids;
+        semaphore _available = { front_ring::nr_ents };
     private:
         std::array<gntref, front_ring<T>::nr_ents> _entries;
         front_ring<T> *_ring;
     public:
-        entries(front_ring<T> *ring) : _ids(front_ring<T>::nr_ents), _ring(ring) {
+        entries(front_ring<T> *ring) : _ring(ring) {
             for (unsigned i = 0; i < front_ring<T>::nr_ents; ++i) {
                 _ids.push(std::move(i));
             }
@@ -86,7 +87,7 @@ public:
         gntref& operator[](std::size_t i) { return _entries[_ring->idx(i)]; }
         friend front_ring;
         future<unsigned> get_index();
-        future<> free_index(unsigned index);
+        void free_index(unsigned index);
     };
 protected:
     uint32_t idx(int i) { return i & (nr_ents - 1); }