docs: Add instructions for heap profiling

Fixes: #5074

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

docs/heap-profiling.md

@@ -0,0 +1,41 @@
+# Heap profiling
+
+Cloud Hypervisor supports generating a profile using
+[dhat](https://docs.rs/dhat/latest/dhat/) of the heap allocations made during
+the runtime of the process.
+
+## Building a suitable binary
+
+This adds the symbol information to the release binary but does not otherwise
+affect the performance.
+
+```
+$ cargo build --profile profiling --features "dhat-heap"
+```
+
+## Generating output
+
+Cloud Hypervisor can then be run as usual. However it is necessary to run with `--seccomp false` as the profiling requires extra syscalls.
+
+```
+$ target/profiling/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 \
+        --seccomp false
+```
+
+When the VMM exits a message like the following will be shown:
+
+```
+dhat: Total:     384,582 bytes in 3,512 blocks
+dhat: At t-gmax: 133,885 bytes in 379 blocks
+dhat: At t-end:  12,160 bytes in 20 blocks
+dhat: The data has been saved to dhat-heap.json, and is viewable with dhat/dh_view.html
+```
+
+The JSON output can then be uploaded to [the dh_view tool](https://nnethercote.github.io/dh_view/dh_view.html) for analysis.
+