FreeBSD/Linux Kernel Cross Reference
sys/net/paravirt.h
1 /*
2 * Copyright (C) 2013 Luigi Rizzo. All rights reserved.
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions
6 * are met:
7 * 1. Redistributions of source code must retain the above copyright
8 * notice, this list of conditions and the following disclaimer.
9 * 2. Redistributions in binary form must reproduce the above copyright
10 * notice, this list of conditions and the following disclaimer in the
11 * documentation and/or other materials provided with the distribution.
12 *
13 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
14 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
15 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
16 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
17 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
18 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
19 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
20 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
21 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
22 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
23 * SUCH DAMAGE.
24 */
25
26 #ifndef NET_PARAVIRT_H
27 #define NET_PARAVIRT_H
28
29 /*
30 * $FreeBSD$
31 *
32 Support for virtio-like communication between host (H) and guest (G) NICs.
33
34 THIS IS EXPERIMENTAL CODE AND SUBJECT TO CHANGE.
35
36 The guest allocates the shared Communication Status Block (csb) and
37 write its physical address at CSBAL and CSBAH (data is little endian).
38 csb->csb_on enables the mode. If disabled, the device acts a regular one.
39
40 Notifications for tx and rx are exchanged without vm exits
41 if possible. In particular (only mentioning csb mode below),
42 the following actions are performed. In the description below,
43 "double check" means verifying again the condition that caused
44 the previous action, and reverting the action if the condition has
45 changed. The condition typically depends on a variable set by the
46 other party, and the double check is done to avoid races. E.g.
47
48 // start with A=0
49 again:
50 // do something
51 if ( cond(C) ) { // C is written by the other side
52 A = 1;
53 // barrier
54 if ( !cond(C) ) {
55 A = 0;
56 goto again;
57 }
58 }
59
60 TX: start from idle:
61 H starts with host_need_txkick=1 when the I/O thread bh is idle. Upon new
62 transmissions, G always updates guest_tdt. If host_need_txkick == 1,
63 G also writes to the TDT, which acts as a kick to H (so pending
64 writes are always dispatched to H as soon as possible.)
65
66 TX: active state:
67 On the kick (TDT write) H sets host_need_txkick == 0 (if not
68 done already by G), and starts an I/O thread trying to consume
69 packets from TDH to guest_tdt, periodically refreshing host_tdh
70 and TDH. When host_tdh == guest_tdt, H sets host_need_txkick=1,
71 and then does the "double check" for race avoidance.
72
73 TX: G runs out of buffers
74 XXX there are two mechanisms, one boolean (using guest_need_txkick)
75 and one with a threshold (using guest_txkick_at). They are mutually
76 exclusive.
77 BOOLEAN: when G has no space, it sets guest_need_txkick=1 and does
78 the double check. If H finds guest_need_txkick== 1 on a write
79 to TDH, it also generates an interrupt.
80 THRESHOLD: G sets guest_txkick_at to the TDH value for which it
81 wants to receive an interrupt. When H detects that TDH moves
82 across guest_txkick_at, it generates an interrupt.
83 This second mechanism reduces the number of interrupts and
84 TDT writes on the transmit side when the host is too slow.
85
86 RX: start from idle
87 G starts with guest_need_rxkick = 1 when the receive ring is empty.
88 As packets arrive, H updates host_rdh (and RDH) and also generates an
89 interrupt when guest_need_rxkick == 1 (so incoming packets are
90 always reported to G as soon as possible, apart from interrupt
91 moderation delays). It also tracks guest_rdt for new buffers.
92
93 RX: active state
94 As the interrupt arrives, G sets guest_need_rxkick = 0 and starts
95 draining packets from the receive ring, while updating guest_rdt
96 When G runs out of packets it sets guest_need_rxkick=1 and does the
97 double check.
98
99 RX: H runs out of buffers
100 XXX there are two mechanisms, one boolean (using host_need_rxkick)
101 and one with a threshold (using host_xxkick_at). They are mutually
102 exclusive.
103 BOOLEAN: when H has no space, it sets host_need_rxkick=1 and does the
104 double check. If G finds host_need_rxkick==1 on updating guest_rdt,
105 it also writes to RDT causing a kick to H.
106 THRESHOLD: H sets host_rxkick_at to the RDT value for which it wants
107 to receive a kick. When G detects that guest_rdt moves across
108 host_rxkick_at, it writes to RDT thus generates a kick.
109 This second mechanism reduces the number of kicks and
110 RDT writes on the receive side when the guest is too slow and
111 would free only a few buffers at a time.
112
113 */
114 struct paravirt_csb {
115 /* XXX revise the layout to minimize cache bounces.
116 * Usage is described as follows:
117 * [GH][RW][+-0] guest/host reads/writes frequently/rarely/almost never
118 */
119 /* these are (mostly) written by the guest */
120 uint32_t guest_tdt; /* GW+ HR+ pkt to transmit */
121 uint32_t guest_need_txkick; /* GW- HR+ G ran out of tx bufs, request kick */
122 uint32_t guest_need_rxkick; /* GW- HR+ G ran out of rx pkts, request kick */
123 uint32_t guest_csb_on; /* GW- HR+ enable paravirtual mode */
124 uint32_t guest_rdt; /* GW+ HR+ rx buffers available */
125 uint32_t guest_txkick_at; /* GW- HR+ tx ring pos. where G expects an intr */
126 uint32_t guest_use_msix; /* GW0 HR0 guest uses MSI-X interrupts. */
127 uint32_t pad[9];
128
129 /* these are (mostly) written by the host */
130 uint32_t host_tdh; /* GR0 HW- shadow register, mostly unused */
131 uint32_t host_need_txkick; /* GR+ HW- start the iothread */
132 uint32_t host_txcycles_lim; /* GW- HR- how much to spin before sleep.
133 * set by the guest */
134 uint32_t host_txcycles; /* GR0 HW- counter, but no need to be exported */
135 uint32_t host_rdh; /* GR0 HW- shadow register, mostly unused */
136 uint32_t host_need_rxkick; /* GR+ HW- flush rx queued packets */
137 uint32_t host_isr; /* GR* HW* shadow copy of ISR */
138 uint32_t host_rxkick_at; /* GR+ HW- rx ring pos where H expects a kick */
139 uint32_t vnet_ring_high; /* Vnet ring physical address high. */
140 uint32_t vnet_ring_low; /* Vnet ring physical address low. */
141 };
142
143 #define NET_PARAVIRT_CSB_SIZE 4096
144 #define NET_PARAVIRT_NONE (~((uint32_t)0))
145
146 #ifdef QEMU_PCI_H
147
148 /*
149 * API functions only available within QEMU
150 */
151
152 void paravirt_configure_csb(struct paravirt_csb** csb, uint32_t csbbal,
153 uint32_t csbbah, QEMUBH* tx_bh, AddressSpace *as);
154
155 #endif /* QEMU_PCI_H */
156
157 #endif /* NET_PARAVIRT_H */
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