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

Zayd Qumsieh
2024-05-02T01:11:11Z

> On Fri, 19 Apr 2024 17:58:09 +0100,
> Will Deacon <will@kernel.org> 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).
> >
> > These microVMs sound quite interesting. What exactly are they? Are you
> > running them under KVM?
> >
> > Ignoring the mechanism for the time being, would it solve your problem
> > if you were able to run specific microVMs in TSO mode, or do you *really*
> > need the VM to have finer-grained control than that? If the whole VM is
> > running in TSO mode, then my concerns largely disappear, as that's
> > indistinguishable from running on a hardware implementation that happens
> > to be TSO.
>
> Since KVM has been mentioned a few times, I'll give my take on this.
>
> Since day 1, it was a conscious decision for KVM/arm64 to emulate the
> architecture, and only that -- this is complicated enough. Meaning
> that no implementation-defined features should be explicitly exposed
> to the guest. So I have no plan to expose any such feature for
> userspace to configure TSO or anything else of the sort.

Agreed. We do not intend for TSO mode to be used extensively for EL1, the
intention is for TSO mode to be reserved for userspace applications that
request it.