docs: Add instructions for using perf

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

docs/profiling.md

@@ -0,0 +1,45 @@
+# Profiling
+
+`perf` can be used to profile the `cloud-hypervisor` binary but it is necessary to make some modifications to the the build in order to produce a binary that gives useful results.
+
+## Building a suitable binary
+
+Modify the `Cargo.toml` file to add `debug = 1` to the `[profile.release]` block. It should look like this:
+
+```
+[profile.release]
+lto = true
+debug = 1
+```
+
+This adds the symbol information to the release binary but does not otherwise affect the performance.
+
+The binary must also be built with frame pointers included so that the call graph can be captured by the profiler.
+
+```
+$ cargo clean && RUSTFLAGS='-C force-frame-pointers=y' cargo build --release
+```
+
+## Profiling
+
+`perf` may then be used in the usual manner:
+
+e.g.
+
+```
+$ perf record -g target/release/cloud-hypervisor \
+        --kernel ~/src/linux/vmlinux \
+        --pmem file=~/workloads/focal.raw \
+        --cpus boot=1 --memory size=1G \
+        --cmdline "root=/dev/pmem0p1 console=ttyS0" \
+        --serial tty --console off \
+        --api-socket=/tmp/api1
+```
+
+For analysing the samples:
+
+```
+$ perf report -g
+```
+
+If profiling with a network device attached either the TAP device must be already created and configured or the profiling must be done as root so that the TAP device can be created.