docs: Add GDB feature document

This commit adds a short GDB feature document.

Signed-off-by: Akira Moroo <retrage01@gmail.com>

docs/gdb.md

@@ -0,0 +1,47 @@
+# GDB Support
+
+This feature allows remote guest debugging using GDB. Note that this feature is only supported on x86_64/KVM.
+
+To enable debugging with GDB, build with the `gdb` feature enabled:
+
+```bash
+cargo build --features gdb
+```
+
+To use the `--gdb` option, specify the Unix Domain Socket with `--path` that Cloud Hypervisor will use to communicate with the host's GDB:
+
+```bash
+./cloud-hypervisor \
+    --kernel hypervisor-fw \
+    --disk path=bionic-server-cloudimg-amd64.raw \
+    --cpus boot=1 \
+    --memory size=1024M \
+    --net "tap=,mac=,ip=,mask=" \
+    --console off \
+    --serial tty \
+    --gdb path=/tmp/ch-gdb-sock
+```
+
+Cloud Hypervisor will listen for GDB on the host side before starting the guest.
+On the host side, connect to the GDB remote server as follows:
+
+```bash
+gdb -q
+(gdb) target remote /tmp/ch-gdb-sock
+Remote debugging using /tmp/ch-gdb-sock
+warning: No executable has been specified, and target does not support
+determining executable automatically. Try using the "file" command.
+0x000000000011217e in ?? ()
+```
+
+You can set up to four hardware breakpoints using the x86 debug register:
+
+```bash
+(gdb) hb *0x1121b7
+Hardware assisted breakpoint 1 at 0x1121b7
+(gdb) c
+Continuing.
+
+Breakpoint 1, 0x00000000001121b7 in ?? ()
+(gdb)
+```