[PATCH v5 12/13] um: nommu: add documentation of nommu UML

[Hajime Tazaki <thehajime@gmail.com>]

This commit adds an initial documentation for !MMU mode of UML.

Signed-off-by: Hajime Tazaki <thehajime@gmail.com>
---
 Documentation/virt/uml/nommu-uml.rst | 177 +++++++++++++++++++++++++++
 MAINTAINERS                          |   1 +
 2 files changed, 178 insertions(+)
 create mode 100644 Documentation/virt/uml/nommu-uml.rst

diff --git a/Documentation/virt/uml/nommu-uml.rst b/Documentation/virt/uml/nommu-uml.rst
new file mode 100644
index 000000000000..a98bfd9d2f38
--- /dev/null
+++ b/Documentation/virt/uml/nommu-uml.rst
@@ -0,0 +1,177 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+UML has been built with CONFIG_MMU since day 0.  The patchset
+introduces the nommu mode on UML in a different angle from what Linux
+Kernel Library tried.
+
+.. contents:: :local:
+
+What is it for ?
+================
+
+- Alleviate syscall hook overhead implemented with ptrace(2)
+- To exercises nommu code over UML (and over KUnit)
+- Less dependency to host facilities
+
+
+How it works ?
+==============
+
+To illustrate how this feature works, the below shows how syscalls are
+called under nommu/UML environment.
+
+- boot kernel, install seccomp filter if ``syscall`` instructions are
+  called from userspace memory based on the address of instruction
+  pointer
+- (userspace starts)
+- calls ``vfork``/``execve`` syscalls
+- ``SIGSYS`` signal raised, handler calls syscall entry point ``__kernel_vsyscall``
+- call handler function in ``sys_call_table[]`` and follow how UML syscall
+  works.
+- return to userspace
+
+
+What are the differences from MMU-full UML ?
+============================================
+
+The current nommu implementation adds 3 different functions which
+MMU-full UML doesn't have:
+
+- kernel address space can directly be accessible from userspace
+  - so, ``uaccess()`` always returns 1
+  - generic implementation of memcpy/strcpy/futex is also used
+- alternate syscall entrypoint without ptrace
+- alternate syscall hook
+  - hook syscall by seccomp filter
+
+With those modifications, it allows us to use unmodified userspace
+binaries with nommu UML.
+
+
+History
+=======
+
+This feature was originally introduced by Ricardo Koller at Open
+Source Summit NA 2020, then integrated with the syscall translation
+functionality with the clean up to the original code.
+
+Building and run
+================
+
+::
+
+   make ARCH=um x86_64_nommu_defconfig
+   make ARCH=um
+
+will build UML with ``CONFIG_MMU=n`` applied.
+
+Kunit tests can run with the following command::
+
+   ./tools/testing/kunit/kunit.py run --kconfig_add CONFIG_MMU=n
+
+To run a typical Linux distribution, we need nommu-aware userspace.
+We can use a stock version of Alpine Linux with nommu-built version of
+busybox and musl-libc.
+
+
+Preparing root filesystem
+=========================
+
+nommu UML requires to use a specific standard library which is aware
+of nommu kernel.  We have tested custom-build musl-libc and busybox,
+both of which have built-in support for nommu kernels.
+
+There are no available Linux distributions for nommu under x86_64
+architecture, so we need to prepare our own image for the root
+filesystem.  We use Alpine Linux as a base distribution and replace
+busybox and musl-libc on top of that.  The following are the step to
+prepare the filesystem for the quick start::
+
+     container_id=$(docker create ghcr.io/thehajime/alpine:3.20.3-um-nommu)
+     docker start $container_id
+     docker wait $container_id
+     docker export $container_id > alpine.tar
+     docker rm $container_id
+
+     mnt=$(mktemp -d)
+     dd if=/dev/zero of=alpine.ext4 bs=1 count=0 seek=1G
+     sudo chmod og+wr "alpine.ext4"
+     yes 2>/dev/null | mkfs.ext4 "alpine.ext4" || true
+     sudo mount "alpine.ext4" $mnt
+     sudo tar -xf alpine.tar -C $mnt
+     sudo umount $mnt
+
+This will create a file image, ``alpine.ext4``, which contains busybox
+and musl with nommu build on the Alpine Linux root filesystem.  The
+file can be specified to the argument ``ubd0=`` to the UML command line::
+
+  ./vmlinux ubd0=./alpine.ext4 rw mem=1024m loglevel=8 init=/sbin/init
+
+We plan to upstream apk packages for busybox and musl so that we can
+follow the proper procedure to set up the root filesystem.
+
+
+Quick start with docker
+=======================
+
+There is a docker image that you can quickly start with a simple step::
+
+  docker run -it -v /dev/shm:/dev/shm --rm ghcr.io/thehajime/alpine:3.20.3-um-nommu
+
+This will launch a UML instance with an pre-configured root filesystem.
+
+Benchmark
+=========
+
+The below shows an example of performance measurement conducted with
+lmbench and (self-crafted) getpid benchmark (with v6.12-rc2 uml/next
+tree).
+
+.. csv-table:: lmbench (usec)
+  :header: ,native,um,um-nommu(s)
+
+  select-10    ,0.5544,29.7143,2.8920
+  select-100   ,2.3992,27.7262,3.7794
+  select-1000  ,20.4708,42.0885,12.6920
+  syscall      ,0.1734,26.2471,2.6070
+  read         ,0.3433,29.8828,2.6923
+  write        ,0.2866,25.9753,2.6925
+  stat         ,1.9195,40.1164,3.1813
+  open/close   ,3.8657,63.4730,6.2049
+  fork+sh      ,1161.1111,5216.5000,462.3077
+  fork+execve  ,536.5263,2117.0000,131.0633
+
+.. csv-table:: do_getpid bench (nsec)
+  :header: ,native,um,um-nommu(s)
+
+  getpid,  172 , 26807 , 2614
+
+Limitations
+===========
+
+generic nommu limitations
+-------------------------
+Since this port is a kernel of nommu architecture so, the
+implementation inherits the characteristics of other nommu kernels
+(riscv, arm, etc), described below.
+
+- vfork(2) should be used instead of fork(2)
+- ELF loader only loads PIE (position independent executable) binaries
+- processes share the address space among others
+- mmap(2) offers a subset of functionalities (e.g., unsupported
+  MMAP_FIXED)
+
+Thus, we have limited options to userspace programs.  We have tested
+Alpine Linux with musl-libc, which has a support nommu kernel.
+
+supported architecture
+----------------------
+The current implementation of nommu UML only works on x86_64 SUBARCH.
+We have not tested with 32-bit environment.
+
+
+Further readings about NOMMU UML
+================================
+
+- NOMMU UML (original code by Ricardo Koller)
+ - https://static.sched.com/hosted_files/ossna2020/ec/kollerr_linux_um_nommu.pdf
diff --git a/MAINTAINERS b/MAINTAINERS
index a097afd76ded..aaffff989580 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -24186,6 +24186,7 @@ USER-MODE LINUX (UML)
 M:	Richard Weinberger <richard@nod.at>
 M:	Anton Ivanov <anton.ivanov@cambridgegreys.com>
 M:	Johannes Berg <johannes@sipsolutions.net>
+M:	Hajime Tazaki <thehajime@gmail.com>
 L:	linux-um@lists.infradead.org
 S:	Maintained
 W:	http://user-mode-linux.sourceforge.net
-- 
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