[RFC PATCH v2 12/13] um: nommu: add documentation of nommu UML
Hajime Tazaki
thehajime at gmail.com
Sun Nov 10 22:27:12 PST 2024
This commit adds an initial documentation for !MMU mode of UML.
Signed-off-by: Hajime Tazaki <thehajime at gmail.com>
---
Documentation/virt/uml/nommu-uml.rst | 221 +++++++++++++++++++++++++++
1 file changed, 221 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
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+.. 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, setup zpoline trampoline code (detailed later) at address 0x0
+- (userspace starts)
+- calls vfork/execve syscalls
+- during execve, more specifically during load_elf_fdpic_binary()
+ function, kernel translates `syscall/sysenter` instructions with `call
+ *%rax`, which usually point to address 0 to NR_syscalls (around
+ 512), where trampoline code was installed during startup.
+- when syscalls are issued by userspace, it jumps to `*%rax`, slides
+ until `nop` instructions end, and jump to hooked function,
+ `__kernel_vsyscall`, which is an entrypoint for syscall under nommu
+ UML environment.
+- 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
+- translation of syscall/sysenter instructions to a trampoline code
+ and syscall hooks
+
+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 eth0=tuntap,tap100,0e:fd:0:0:0:1,172.17.0.1 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).
+
+### lmbench (usec)
+
+||native|um|um-nommu|
+|--|--|--|--|
+|select-10 |0.5644|31.0917|0.2743|
+|select-100 |2.3869|31.4651|1.1472|
+|select-1000 |20.4004|36.4966|9.7533|
+|syscall |0.1733|25.9904|0.1053|
+|read |0.3438|27.4873|0.1451|
+|write |0.2862|25.8794|0.1361|
+|stat |1.9250|37.5072|0.4532|
+|open/close |3.8961|65.1736|0.7665|
+|fork+sh |1173.8889|5404.5000|20577.0000|
+|fork+execve |535.2105|2179.2000|4716.3333|
+
+### do_getpid bench (nsec)
+
+||native|um|um-nommu|
+|--|--|--|--|
+|getpid | 172 | 25602 | 103|
+
+
+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.
+
+access to mmap_min_addr
+----------------------
+As the mechanism of syscall translations relies on an ability to
+write/read memory address zero (0x0), we need to configure host kernel
+with the following command:
+
+```
+% sh -c "echo 0 > /proc/sys/vm/mmap_min_addr"
+```
+
+supported architecture
+----------------------
+The current implementation of nommu UML only works on x86_64 SUBARCH.
+We have not tested with 32-bit environment.
+
+target of syscall translation
+-----------------------------
+The syscall translation only applies to the executable and interpreter
+of ELF binary files which are processed by execve(2) syscall for the
+moment: other libraries such as linked library and dlopen-ed one
+aren't translated; we may be able to trigger the translation by
+LD_PRELOAD. JIT compiler generated code is also generated after execve
+thus, it is not currently translated.
+
+Note that with musl-libc in Alpine Linux which we've been tested, most
+of syscalls are implemented in the interpreter file
+(ld-musl-x86_64.so) and calling syscall/sysenter instructions from the
+linked/loaded libraries might be rare. But it is definitely possible
+so, a workaround with LD_PRELOAD is effective.
+
+
+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
+
+- zpoline: syscall translation mechanism
+https://www.usenix.org/conference/atc23/presentation/yasukata
--
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