selftests/bpf: Add description for running vmtest on RV64

Add description in tools/testing/selftests/bpf/README.rst
for running vmtest on RV64.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20240905081401.1894789-11-pulehui@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/README.rst

@@ -85,6 +85,38 @@ In case of linker errors when running selftests, try using static linking:
           If you want to change pahole and llvm, you can change `PATH` environment
           variable in the beginning of script.
 
+Running vmtest on RV64
+======================
+To speed up testing and avoid various dependency issues, it is recommended to
+run vmtest in a Docker container. Before running vmtest, we need to prepare
+Docker container and local rootfs image. The overall steps are as follows:
+
+1. Create Docker container as shown in link [0].
+
+2. Use mkrootfs_debian.sh script [1] to build local rootfs image:
+
+.. code-block:: console
+
+  $ sudo ./mkrootfs_debian.sh --arch riscv64 --distro noble
+
+3. Start Docker container [0] and run vmtest in the container:
+
+.. code-block:: console
+
+  $ PLATFORM=riscv64 CROSS_COMPILE=riscv64-linux-gnu- \
+    tools/testing/selftests/bpf/vmtest.sh \
+    -l <path of local rootfs image> -- \
+    ./test_progs -d \
+        \"$(cat tools/testing/selftests/bpf/DENYLIST.riscv64 \
+            | cut -d'#' -f1 \
+            | sed -e 's/^[[:space:]]*//' \
+                  -e 's/[[:space:]]*$//' \
+            | tr -s '\n' ',' \
+        )\"
+
+Link: https://github.com/pulehui/riscv-bpf-vmtest.git [0]
+Link: https://github.com/libbpf/ci/blob/main/rootfs/mkrootfs_debian.sh [1]
+
 Additional information about selftest failures are
 documented here.