Re: [RFC PATCH 0/5] RISC-V: Add dynamic TSO support

From: Peter Zijlstra <peterz@infradead.org>
To: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Cc: "Christoph Muellner" <christoph.muellner@vrull.eu>,
	linux-riscv@lists.infradead.org, linux-kselftest@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Shuah Khan" <shuah@kernel.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Anup Patel" <apatel@ventanamicro.com>,
	"Philipp Tomsich" <philipp.tomsich@vrull.eu>,
	"Andrew Jones" <ajones@ventanamicro.com>,
	"Guo Ren" <guoren@kernel.org>,
	"Daniel Henrique Barboza" <dbarboza@ventanamicro.com>,
	"Conor Dooley" <conor.dooley@microchip.com>,
	"Björn Töpel" <bjorn@rivosinc.com>,
	"Alan Stern" <stern@rowland.harvard.edu>,
	"Andrea Parri" <parri.andrea@gmail.com>,
	"Will Deacon" <will@kernel.org>,
	"Daniel Lustig" <dlustig@nvidia.com>
Subject: Re: [RFC PATCH 0/5] RISC-V: Add dynamic TSO support
Date: Fri, 24 Nov 2023 12:54:30 +0100	[thread overview]
Message-ID: <20231124115430.GS3818@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <59da3e41-abb3-405a-8f98-c74bdf26935b@huaweicloud.com>

On Fri, Nov 24, 2023 at 12:04:09PM +0100, Jonas Oberhauser wrote:

> > I think ARM64 approached this problem by adding the
> > load-acquire/store-release instructions and for TSO based code,
> > translate into those (eg. x86 -> arm64 transpilers).
> 
> 
> Although those instructions have a bit more ordering constraints.
> 
> I have heard rumors that the apple chips also have a register that can be
> set at runtime.

Oh, I thought they made do with the load-acquire/store-release thingies.
But to be fair, I haven't been paying *that* much attention to the apple
stuff.

I did read about how they fudged some of the x86 flags thing.

> And there are some IBM machines that have a setting, but not sure how it is
> controlled.

Cute, I'm assuming this is the Power series (s390 already being TSO)? I
wasn't aware they had this.

> > IIRC Risc-V actually has such instructions as well, so *why* are you
> > doing this?!?!
> 
> 
> Unfortunately, at least last time I checked RISC-V still hadn't gotten such
> instructions.
> What they have is the *semantics* of the instructions, but no actual opcodes
> to encode them.

Well, that sucks..

> I argued for them in the RISC-V memory group, but it was considered to be
> outside the scope of that group.
> 
> Transpiling with sufficient DMB ISH to get the desired ordering is really
> bad for performance.

Ha!, quite dreadful I would imagine.

> That is not to say that linux should support this. Perhaps linux should
> pressure RISC-V into supporting implicit barriers instead.

I'm not sure I count for much in this regard, but yeah, that sounds like
a plan :-)