Re: [PATCH 0/4] arm64: Support the TSO memory model

[Eric Curtin]

On Mon, 6 May 2024 at 17:13, Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Mon, 06 May 2024 12:21:40 +0100,
> Sergio Lopez Pascual <slp@xxxxxxxxxx> wrote:
> >
> > Eric Curtin <ecurtin@xxxxxxxxxx> writes:
> >
> > > On Fri, 19 Apr 2024 at 18:08, Will Deacon <will@xxxxxxxxxx> wrote:
> > >>
> > >> On Thu, Apr 11, 2024 at 11:19:13PM +0900, Hector Martin wrote:
> > >> > On 2024/04/11 22:28, Will Deacon wrote:
> > >> > >   * Some binaries in a distribution exhibit instability which goes away
> > >> > >     in TSO mode, so a taskset-like program is used to run them with TSO
> > >> > >     enabled.
> > >> >
> > >> > Since the flag is cleared on execve, this third one isn't generally
> > >> > possible as far as I know.
> > >>
> > >> Ah ok, I'd missed that. Thanks.
> > >>
> > >> > > In all these cases, we end up with native arm64 applications that will
> > >> > > either fail to load or will crash in subtle ways on CPUs without the TSO
> > >> > > feature. Assuming that the application cannot be fixed, a better
> > >> > > approach would be to recompile using stronger instructions (e.g.
> > >> > > LDAR/STLR) so that at least the resulting binary is portable. Now, it's
> > >> > > true that some existing CPUs are TSO by design (this is a perfectly
> > >> > > valid implementation of the arm64 memory model), but I think there's a
> > >> > > big difference between quietly providing more ordering guarantees than
> > >> > > software may be relying on and providing a mechanism to discover,
> > >> > > request and ultimately rely upon the stronger behaviour.
> > >> >
> > >> > The problem is "just" using stronger instructions is much more
> > >> > expensive, as emulators have demonstrated. If TSO didn't serve a
> > >> > practical purpose I wouldn't be submitting this, but it does. This is
> > >> > basically non-negotiable for x86 emulation; if this is rejected
> > >> > upstream, it will forever live as a downstream patch used by the entire
> > >> > gaming-on-Mac-Linux ecosystem (and this is an ecosystem we are very
> > >> > explicitly targeting, given our efforts with microVMs for 4K page size
> > >> > support and the upcoming Vulkan drivers).
> >
> > In addition to the use case Hector exposed here, there's another,
> > potentially larger one, which is running x86_64 containers on aarch64
> > systems, using a combination of both Virtualization and emulation.
> >
> > In this scenario, both not being able to use TSO for emulation
> > and having to enable it all the time for the whole VM have a very large
> > impact on performance (~25% on some workloads).
>
> Well, there is always a price to pay somewhere, and this is the usual
> trade-off between performance and maintainability.
>
> > I understand the concern about the risk of userspace fragmentation, but
> > I was wondering if we could minimize it to an acceptable level by
> > narrowing down the context. For instance, since both use cases we're
> > bringing to the table imply the use of Virtualization, we should be able
> > to restrict PR_SET_MEM_MODEL to only be accepted when running on EL1
> > (and not in nVHE nor pKVM), returning EINVAL otherwise. This would
> > heavily discourage users from relying on this feature for native
> > applications that can run on arbitrary contexts, hence drastically
> > reducing the fragmentation risk.
>
> As I explained in another sub-thread[1], I am not prepared to allow
> non architectural state to be exposed to a guest.  I'm also not
> prepared to make significant ABI differences between VHE, nVHE, hVHE,
> with or without pKVM, because the job of the kernel is to abstract
> those differences.
>
> > We would still need a way to ensure the trap gets to the VMM and for
> > the VMM to operate on the impdef ACTLR_EL12, but that should be dealt on
> > a different series.
>
> The VMM can't use ACTLR_EL12, by the very definition of this register
> (the clue is in the name).  You'd have to proxy the write in the
> kernel and context-switch it, which means adding non-architectural
> state to KVM, breaking VM migration and adding more kludges to the
> existing Apple-specific host crap.
>
> Also, let's realise that we are talking about making significant
> changes to the arm64 ABI for a platform that is still not fully
> supported in the upstream kernel. I have the feeling that changing the

Note there's two use-cases for this today, bare-metal Linux on Apple
Silicon devices and Linux VMs on macOS. The latter is fully supported
in the upstream kernel.

Apple Silicon devices have a significantly sized Linux userbase as
there is a shortage of decent local ARM development machines for Linux
as well as just being decent local laptop/desktop SoC's in general for
AI. The general performance of the SoC makes it very useful.

Is mise le meas/Regards,

Eric Curtin

> memory model dynamically may not be of the utmost priority until then.
>
> Thanks,
>
>         M.
>
> [1] https://lore.kernel.org/all/867cgcqrb9.wl-maz@xxxxxxxxxx
>
> --
> Without deviation from the norm, progress is not possible.
>