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Date: Fri, 19 Apr 2024 19:05:12 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: Will Deacon <will@kernel.org>
Cc: Zayd Qumsieh <zayd_qumsieh@apple.com>, Hector Martin <marcan@marcan.st>,
	Marc Zyngier <maz@kernel.org>, Mark Rutland <mark.rutland@arm.com>,
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	Asahi Linux <asahi@lists.linux.dev>
Subject: Re: [PATCH 0/4] arm64: Support the TSO memory model
Message-ID: <ZiKyWGKTw6Aqntod@arm.com>
References: <f6484dcd-ebf6-4b6f-be17-69b05539e33b@marcan.st>
 <20240416022242.89623-1-zayd_qumsieh@apple.com>
 <20240419165826.GB4020@willie-the-truck>
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In-Reply-To: <20240419165826.GB4020@willie-the-truck>

On Fri, Apr 19, 2024 at 05:58:26PM +0100, Will Deacon wrote:
> On Mon, Apr 15, 2024 at 07:22:41PM -0700, Zayd Qumsieh wrote:
> > >I'm probably going to make myself hugely unpopular here, but I have a
> > >strong objection to this patch series as it stands. I firmly believe
> > >that providing a prctl() to query and toggle the memory model to/from
> > >TSO is going to lead to subtle fragmentation of arm64 Linux userspace.
> > 
> > It's definitely not our intent to fragment the ecosystem. The goal
> > of this memory ordering is to simplify emulation layers that benefit
> > from this. If you have suggestions to reduce the risk of it being
> > misused outside of emulators, we'd be happy to look into it.
> 
> Once you have exposed this toggle via prctl(), it doesn't really matter
> what your intentions where. It will get used outside of emulation laters
> and we'll be stuck supporting it.

Just FTR, I fully agree with Will. I'm strongly against this kind of ABI
for a non-architected, implementation defined feature. I can't even tell
exactly what TSO means on the Apple hardware. Is it close to the x86
TSO? Is there a formal memory model for it? Are future Apple (or other
Arm vendor) implementations going to follow exactly the same model to be
able to call it some form of "Apple standard" that deserves an ABI?

So, sorry, I'm going to NAK these approaches proposing imp def features
as generic opt-in mechanisms (the microVMs thing sounds doable though,
to my limited understanding; I guess that would mean running the
emulator in a VM).

-- 
Catalin