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From: Eric Curtin <ecurtin@redhat.com>
Date: Sat, 20 Apr 2024 13:13:38 +0100
Message-ID: <CAOgh=Fykg3Xb8Y59R_tJ7_jXe-ozXRMQjU+qVy5DdmFn3pkcPw@mail.gmail.com>
Subject: Re: [PATCH 0/4] arm64: Support the TSO memory model
To: Will Deacon <will@kernel.org>
Cc: Hector Martin <marcan@marcan.st>, Catalin Marinas <catalin.marinas@arm.com>, 
	Marc Zyngier <maz@kernel.org>, Mark Rutland <mark.rutland@arm.com>, 
	Zayd Qumsieh <zayd_qumsieh@apple.com>, Justin Lu <ih_justin@apple.com>, 
	Ryan Houdek <Houdek.Ryan@fex-emu.org>, Mark Brown <broonie@kernel.org>, 
	Ard Biesheuvel <ardb@kernel.org>, Mateusz Guzik <mjguzik@gmail.com>, 
	Anshuman Khandual <anshuman.khandual@arm.com>, Oliver Upton <oliver.upton@linux.dev>, 
	Miguel Luis <miguel.luis@oracle.com>, Joey Gouly <joey.gouly@arm.com>, 
	Christoph Paasch <cpaasch@apple.com>, Kees Cook <keescook@chromium.org>, 
	Sami Tolvanen <samitolvanen@google.com>, Baoquan He <bhe@redhat.com>, 
	Joel Granados <j.granados@samsung.com>, Dawei Li <dawei.li@shingroup.cn>, 
	Andrew Morton <akpm@linux-foundation.org>, Florent Revest <revest@chromium.org>, 
	David Hildenbrand <david@redhat.com>, Stefan Roesch <shr@devkernel.io>, Andy Chiu <andy.chiu@sifive.com>, 
	Josh Triplett <josh@joshtriplett.org>, Oleg Nesterov <oleg@redhat.com>, Helge Deller <deller@gmx.de>, 
	Zev Weiss <zev@bewilderbeest.net>, Ondrej Mosnacek <omosnace@redhat.com>, 
	Miguel Ojeda <ojeda@kernel.org>, linux-arm-kernel@lists.infradead.org, 
	linux-kernel@vger.kernel.org, Asahi Linux <asahi@lists.linux.dev>, 
	Sergio Lopez Pascual <slp@redhat.com>
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On Fri, 19 Apr 2024 at 18:08, 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?

It's the magic of libkrun. This is one of the git repos in the family
of libkrun, it has a wide array of use cases, which I personally won't
do much justice explaining all then, this is just one
repo/tool/usecases:

https://github.com/containers/krunvm

https://sinrega.org/running-microvms-on-m1/

CC'ing @Sergio Lopez Pascual the lead of krun in general.

Is mise le meas/Regards,

Eric Curtin

>
> 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.
>
> > That said, I have a pragmatic proposal here. The "fixed TSO" part of the
> > implementation should be harmless, since those CPUs would correctly run
> > poorly-written applications anyway so the API is moot. That leaves Apple
> > Silicon. Our native kernels are and likely always will be 16K page size,
> > due to a bunch of pain around 16K-only IOMMUs (4K kernels do boot
> > natively but with very broken functionality including no GPU
> > acceleration) plus performance differences that favor 16K. How about we
> > gate the TSO functionality to only be supported on 4K kernel builds?
> > This would make them only work in 4K VMs on Asahi Linux. We are very
> > explicitly discouraging people from trying to use the microVMs to work
> > around page size problems (which they can already do, another
> > fragmentation problem, anyway); any application which requires the 4K VM
> > to run that isn't an emulator is already clearly broken and advertising
> > that fact openly. So, adding TSO to this should be only a marginal risk
> > of further fragmentation, and it wouldn't allow apps to "sneakily" "just
> > work" on Apple machines by abusing TSO.
>
> I appreciate that you're trying to be constructive here, but I don't think
> we should tie this to the page size. It's an artifical limitation and I
> don't think it really addresses the underlying concerns that I have.
>
> Will
>