This commit adds an initial documentation for !MMU mode of UML. Signed-off-by: Hajime Tazaki <thehaj...@gmail.com> --- Documentation/virt/uml/nommu-uml.rst | 219 +++++++++++++++++++++++++++ 1 file changed, 219 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..3171f34a50bd --- /dev/null +++ b/Documentation/virt/uml/nommu-uml.rst @@ -0,0 +1,219 @@ +.. SPDX-License-Identifier: GPL-2.0 + +UML has been built with CONFIG_MMU since day 0. The feature +introduces the nommu mode 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-rc3 linus tree). + +### lmbench (usec) + +||native|um|um-nommu| +|--|--|--|--| +|select-10 |0.5645|28.3738|0.2647| +|select-100 |2.3872|28.8385|1.1021| +|select-1000 |20.5527|37.6364|9.4264| +|syscall |0.1735|26.8711|0.1037| +|read |0.3442|28.5771|0.1370| +|write |0.2862|28.7340|0.1236| +|stat |1.9236|38.5928|0.4640| +|open/close |3.8308|66.8451|0.7789| +|fork+sh |1176.4444|8221.5000|21443.0000| +|fork+execve |533.1053|3034.5000|4894.3333| + +### do_getpid bench (nsec) + +||native|um|um-nommu| +|--|--|--|--| +|getpid | 180 | 31579 | 101| + + +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. + +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 -- 2.43.0
