[RFC PATCH 0/4] um: Add SMP support

[Tiwei Bie]

From: Tiwei Bie <tiwei.btw at antgroup.com>

This is a follow-up to the discussion in this thread:
https://lore.kernel.org/linux-um/20250711065021.2535362-4-tiwei.bie@linux.dev/

Currently, this is still a PoC and requires further improvements.

I performed a kernel build test inside UML with 8 virtual CPUs enabled:

$ tar xf linux-6.15.tar.gz
$ cd linux-6.15
$ make ARCH=um defconfig > /dev/null
$ time make ARCH=um -j16 > /dev/null

real    6m2.529s
user    43m37.665s
sys     0m34.585s
$ ./linux --version
6.15.0
$ cat /proc/cpuinfo | grep -c '^processor'
8

Here are some steps to try out SMP support in UML:

1. Build UML with CONFIG_SMP=y and, for example, CONFIG_NR_CPUS=8.

2. Launch a UML instance with, for example, 8 virtual CPUs.

 $ ./linux mem=16G ncpus=8 nohz=n seccomp=on init=/bin/sh \
           ubd0=$your_rootfs_image

This patchset depends on the following patchset:
https://lore.kernel.org/linux-um/20250711065021.2535362-1-tiwei.bie@linux.dev/

Tiwei Bie (4):
  um: Stop tracking virtual CPUs via mm_cpumask()
  um: Remove unused cpu_data and current_cpu_data macros
  um: vdso: Implement __vdso_getcpu() via syscall
  um: Add SMP support

 arch/um/Kconfig                         |  28 ++-
 arch/um/include/asm/Kbuild              |   3 +
 arch/um/include/asm/current.h           |   5 +-
 arch/um/include/asm/hardirq.h           |  24 ++-
 arch/um/include/asm/irqflags.h          |   4 +-
 arch/um/include/asm/mmu.h               |   7 +
 arch/um/include/asm/mmu_context.h       |  11 --
 arch/um/include/asm/pgtable.h           |   2 +
 arch/um/include/asm/processor-generic.h |   8 +-
 arch/um/include/asm/smp.h               |  31 +++-
 arch/um/include/asm/spinlock.h          |   8 +
 arch/um/include/linux/smp-internal.h    |   8 +
 arch/um/include/linux/time-internal.h   |   3 +
 arch/um/include/shared/kern_util.h      |   2 +
 arch/um/include/shared/longjmp.h        |   3 +-
 arch/um/include/shared/os.h             |  12 +-
 arch/um/include/shared/smp.h            |  14 ++
 arch/um/kernel/Makefile                 |   1 +
 arch/um/kernel/irq.c                    |  31 +++-
 arch/um/kernel/ksyms.c                  |   2 +-
 arch/um/kernel/mem.c                    |   2 +
 arch/um/kernel/process.c                |  19 +-
 arch/um/kernel/skas/mmu.c               |  16 +-
 arch/um/kernel/smp.c                    | 223 ++++++++++++++++++++++++
 arch/um/kernel/time.c                   |  48 +++--
 arch/um/kernel/tlb.c                    |   5 +-
 arch/um/kernel/trap.c                   |   2 +-
 arch/um/kernel/um_arch.c                |  60 ++++++-
 arch/um/os-Linux/Makefile               |   4 +-
 arch/um/os-Linux/file.c                 |  72 ++++++--
 arch/um/os-Linux/main.c                 |   5 +-
 arch/um/os-Linux/process.c              |  15 ++
 arch/um/os-Linux/signal.c               |  16 +-
 arch/um/os-Linux/skas/process.c         |   1 +
 arch/um/os-Linux/smp.c                  |  44 +++++
 arch/um/os-Linux/start_up.c             |   3 +
 arch/um/os-Linux/time.c                 |  29 +--
 arch/um/os-Linux/user_syms.c            |   5 +
 arch/x86/um/vdso/um_vdso.c              |  18 +-
 39 files changed, 695 insertions(+), 99 deletions(-)
 create mode 100644 arch/um/include/asm/spinlock.h
 create mode 100644 arch/um/include/linux/smp-internal.h
 create mode 100644 arch/um/include/shared/smp.h
 create mode 100644 arch/um/kernel/smp.c
 create mode 100644 arch/um/os-Linux/smp.c

-- 
2.34.1