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

[Marc Zyngier]

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
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.