lwip: add README

Author: lschuermann

examples/lwip_ethernet_tap/README.md

@@ -0,0 +1,66 @@
+# LwIP Tap Ethernet Example
+
+This example demonstrates running a webserver using LwIP, using an Ethernet MAC
+exposed through the [`ethernet_tap`][ethernet-tap-capsule] kernel capsule
+driver.
+
+## Running with the QEMU RISC-V 32 bit board
+
+You can run this app with the QEMU RISC-V 32 bit `virt` board. This board has
+support for QEMU's VirtIO network device, and can use a userspace network stack
+to forward a local host-port to the LwIP network stack running in this
+application.
+
+First, build this application:
+
+```
+libtock-c/examples/lwip_ethernet_tap $ make
+  CC        main.c
+  LD        build/rv32imac/rv32imac.0x80100080.0x80210000.elf
+ E2T        build/lwip_ethernet_tap.tab
+Application size report for arch family rv32i:
+   text    data     bss     dec     hex filename
+  71256     104   48976  120336   1d610 build/rv32imac/rv32imac.0x80100080.0x80210000.elf
+```
+
+Then, in the kernel run the QEMU board with this application:
+```
+tock/boards/qemu_rv32_virt $ make run-app \
+    APP=$LIBTOCK_C/examples/lwip_ethernet_tap/build/rv32imac/rv32imac.0x80100080.0x80210000.tbf \
+	NETDEV=SLIRP NETDEV_SLIRP_ARGS='hostfwd=tcp::8081-192.168.1.50:80'
+    Finished `release` profile [optimized + debuginfo] target(s) in 0.10s
+   text    data     bss     dec     hex filename
+  89088      48   41908  131044   1ffe4 $TOCK/target/riscv32imac-unknown-none-elf/release/qemu_rv32_virt
+
+Running QEMU emulator version 9.1.3 (tested: 7.2.0) with
+  - kernel $TOCK/kernel/target/riscv32imac-unknown-none-elf/release/qemu_rv32_virt.elf
+  - app $LIBTOCK_C/examples/lwip_ethernet_tap/build/rv32imac/rv32imac.0x80100080.0x80210000.tbf
+To exit type C-a x
+
+qemu-system-riscv32 \
+  -machine virt \
+  -semihosting \
+  -global driver=riscv-cpu,property=smepmp,value=true \
+  -global virtio-mmio.force-legacy=false \
+  -device virtio-rng-device \
+  -netdev user,id=n0,net=192.168.1.0/24,dhcpstart=192.168.1.255,hostfwd=tcp::8081-192.168.1.50:80 \
+  -device virtio-net-device,netdev=n0 \
+  -nographic \
+  -bios $TOCK/target/riscv32imac-unknown-none-elf/release/qemu_rv32_virt.elf \
+  -device loader,file=$LIBTOCK_C/examples/lwip_ethernet_tap/build/rv32imac/rv32imac.0x80100080.0x80210000.tbf,addr=0x80100000
+QEMU RISC-V 32-bit "virt" machine, initialization complete.
+- Found VirtIO EntropySource device, enabling RngDriver
+- Found VirtIO NetworkCard device, enabling EthernetTapDriver
+Entering main loop.
+-> userspace tap network app
+-> tapif_init: success
+-> tapif_status: 192.168.1.50
+-> tap interface t0 added
+tock$
+```
+
+Following this, you should be able to navigate to `http://127.0.0.1:8081` and
+see LwIP's default web page.
+
+[ethernet-tap-capsule]: https://github.com/tock/tock/blob/master/capsules/extra/src/ethernet_tap.rs
+[qemu-rv32-board]: https://github.com/tock/tock/tree/master/boards/qemu_rv32_virt