Re: [PATCH 0/3] tso: aarch64: Expose TSO for virtualized linux on Apple Silicon

[Hector Martin]

On 2024/04/11 6:16, Zayd Qumsieh wrote:
> x86 CPUs use a TSO memory model. Apple Silicon CPUs have the ability to
> selectively use a TSO memory model. This can be done by setting the
> ACTLR.TSOEN bit to 1. This feature is useful for x86 emulators, since it
> removes the need for emulators to insert memory barriers in order to abide
> by the TSO memory model. This patch series will add ACTLR.TSOEN support to
> virtualized linux on Apple Silicon machines. Userspace will be able to use
> a prctl to change the memory model of the CPU from the default ARM64 memory
> model to a TSO memory model.
> 
> A simple test can be used to determine if the TSO memory model is in use.
> This must be done on Apple Silicon MacOS Sonoma version 14.4 or later,
> since earlier versions do not support modification of the TSOEN bit.
> https://github.com/saagarjha/TSOEnabler/blob/master/testtso/main.c
> 
> This program will hang indefinitely if TSO is in use, and will crash almost
> immediately if it is not in use.

Well this is unexpected, given I talked to Justin Lu at Apple back in
December and I thought our plan was to work together on the series I've
had cooking in the Asahi tree [1] for a while now, which is actually
shipping in thousands of Asahi Linux systems in production and actually
already supported by the FEX-Emu project with our ABI. You CCed 30+
people, but not me nor the asahi@xxxxxxxxxxxxxxx mailing list...

[1] https://github.com/AsahiLinux/linux/tree/bits/220-tso

Given that we're here now, I'll send out my series for review and see
what people think about that one.

> 
> Zayd Qumsieh (3):
>   tso: aarch64: allow linux kernel to read/write ACTLR.TSOEN
>   tso: aarch64: context-switch tso bit on thread switch
>   tso: aarch64: allow userspace to set tso bit using prctl
> 
>  arch/arm64/Kconfig                 | 19 +++++++++
>  arch/arm64/include/asm/processor.h |  4 ++
>  arch/arm64/include/asm/sysreg.h    |  7 ++++
>  arch/arm64/include/asm/tso.h       | 19 +++++++++
>  arch/arm64/kernel/Makefile         |  2 +-
>  arch/arm64/kernel/process.c        | 61 +++++++++++++++++++++++++++++
>  arch/arm64/kernel/tso.c            | 62 ++++++++++++++++++++++++++++++
>  include/uapi/linux/prctl.h         |  9 +++++
>  kernel/sys.c                       | 11 ++++++
>  9 files changed, 193 insertions(+), 1 deletion(-)
>  create mode 100644 arch/arm64/include/asm/tso.h
>  create mode 100644 arch/arm64/kernel/tso.c
> 

- Hector