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

[Benjamin Berg]

Hi Tiwei,

this is pretty cool, I really did not expect patches for SMP :-)

I have not dived in a lot. From a cursory glance, I expect that there
are currently still issues if a process is actually multi-threaded. But
even this state is really neat already.

As for proper multi-threading support, I would like to make sure you
are aware of the flush_tlb_* functions in tlbflush.h. These will need
to be updated so that these flushes happen synchronously as another
thread might otherwise access data for too long.

There are probably a few things to figure out in that regard about
where/when to flush the TLB entries in a multi-threaded userspace.
After all, there can be up to the number of CPUs worker threads and one
probably only wants one of them to sync the TLBs at any point in tmie.

Anyway, nice work!
Benjamin

On Mon, 2025-07-14 at 01:25 +0800, Tiwei Bie wrote:
> 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
>