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Sat, 08 Nov 2025 00:06:22 -0800 (PST) Received: by ikb-h07-29-noble.in.iijlab.net (Postfix, from userid 1010) id 2C76011388DD; Sat, 8 Nov 2025 17:06:21 +0900 (JST) From: Hajime Tazaki To: linux-um@lists.infradead.org Cc: thehajime@gmail.com, ricarkol@google.com, Liam.Howlett@oracle.com, linux-kernel@vger.kernel.org Subject: [PATCH v13 12/13] um: nommu: add documentation of nommu UML Date: Sat, 8 Nov 2025 17:05:47 +0900 Message-ID: <16940d31af89a3127acf29d23e10dcb9b7b9f4e3.1762588860.git.thehajime@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" This commit adds an initial documentation for !MMU mode of UML. Signed-off-by: Hajime Tazaki --- Documentation/virt/uml/nommu-uml.rst | 180 +++++++++++++++++++++++++++ MAINTAINERS | 1 + 2 files changed, 181 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..f049bbc697d1 --- /dev/null +++ b/Documentation/virt/uml/nommu-uml.rst @@ -0,0 +1,180 @@ +.. 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 ? +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +- 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 ? +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +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_v= syscall`` +- call handler function in ``sys_call_table[]`` and follow how UML syscall + works. +- return to userspace + + +What are the differences from MMU-full UML ? +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +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 +=3D=3D=3D=3D=3D=3D=3D + +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 +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +:: + + make ARCH=3Dum x86_64_nommu_defconfig + make ARCH=3Dum + +will build UML with ``CONFIG_MMU=3Dn`` applied. + +Kunit tests can run with the following command:: + + ./tools/testing/kunit/kunit.py run --kconfig_add CONFIG_MMU=3Dn + +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 +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D + +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=3D$(docker create ghcr.io/thehajime/alpine:3.20.3-um-nom= mu) + docker start $container_id + docker wait $container_id + docker export $container_id > alpine.tar + docker rm $container_id + + mnt=3D$(mktemp -d) + dd if=3D/dev/zero of=3Dalpine.ext4 bs=3D1 count=3D0 seek=3D1G + 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=3D`` to the UML command line:: + + ./vmlinux ubd0=3D./alpine.ext4 rw mem=3D1024m loglevel=3D8 init=3D/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 +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +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 +=3D=3D=3D=3D=3D=3D=3D=3D=3D + +The below shows an example of performance measurement conducted with +lmbench and (self-crafted) getpid benchmark (with v6.17-rc5 uml/next +tree). + +.. csv-table:: lmbench (usec) + :header: ,native,um,um-mmu(s),um-nommu(s) + + select-10 ,0.5319,36.1214,24.2795,2.9174 + select-100 ,1.6019,34.6049,28.8865,3.8080 + select-1000 ,12.2588,43.6838,48.7438,12.7872 + syscall ,0.1644,35.0321,53.2119,2.5981 + read ,0.3055,31.5509,45.8538,2.7068 + write ,0.2512,31.3609,29.2636,2.6948 + stat ,1.8894,43.8477,49.6121,3.1908 + open/close ,3.2973,77.5123,68.9431,6.2575 + fork+sh ,1110.3000,7359.5000,4618.6667,439.4615 + fork+execve ,510.8182,2834.0000,2461.1667,139.7848 + +.. csv-table:: do_getpid bench (nsec) + :header: ,native,um,um-mmu(s),um-nommu(s) + + getpid , 161 , 34477 , 26242 , 2599 + +(um-nommu(s) is with seccomp syscall hook, um-mmu(s) is SECCOMP mode, +respectively) + +Limitations +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +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 +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D + +- NOMMU UML (original code by Ricardo Koller) + - https://static.sched.com/hosted_files/ossna2020/ec/kollerr_linux_um_nom= mu.pdf diff --git a/MAINTAINERS b/MAINTAINERS index 3da2c26a796b..2f227f56d04e 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -26764,6 +26764,7 @@ USER-MODE LINUX (UML) M: Richard Weinberger M: Anton Ivanov M: Johannes Berg +M: Hajime Tazaki L: linux-um@lists.infradead.org S: Maintained W: http://user-mode-linux.sourceforge.net --=20 2.43.0