Re: [sw-dev] SBI extension proposal v2

From: Alexander Graf <agraf@suse.de>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Atish Patra <atish.patra@wdc.com>,
	ron minnich <rminnich@gmail.com>,
	Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Cc: "mark.rutland@arm.com" <mark.rutland@arm.com>,
	Christoph Hellwig <hch@infradead.org>,
	Damien Le Moal <Damien.LeMoal@wdc.com>,
	Olof Johansson <olof.johansson@gmail.com>,
	"alankao@andestech.com" <alankao@andestech.com>,
	"abner.chang@hpe.com" <abner.chang@hpe.com>,
	Anup Patel <anup@brainfault.org>,
	Palmer Dabbelt <palmer@sifive.com>,
	"zong@andestech.com" <zong@andestech.com>,
	Alistair Francis <Alistair.Francis@wdc.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	"mick@ics.forth.gr" <mick@ics.forth.gr>,
	"sw-dev@groups.riscv.org" <sw-dev@groups.riscv.org>,
	"linux-riscv@lists.infradead.org"
	<linux-riscv@lists.infradead.org>,
	Andrew Waterman <andrew@sifive.com>
Subject: Re: [sw-dev] SBI extension proposal v2
Date: Mon, 28 Jan 2019 13:31:41 +0100	[thread overview]
Message-ID: <452be0d3-da8e-643e-9f91-c38f0af36ffd@suse.de> (raw)
In-Reply-To: <d476c0e2b525fb0642b3155a247495cb1850c208.camel@kernel.crashing.org>

On 02.12.18 07:18, Benjamin Herrenschmidt wrote:
> On Sat, 2018-11-10 at 19:58 -0800, Atish Patra wrote:
>>
>> I think the larger picture might have lost somewhere because of the 
>> verbosity of the proposal. The idea of adding vendor extension support 
>> in SBI is to allow them provide a standard mechanism to facilitate this.
>> In absence of that, vendor might adopt different approaches to support 
>> their feature leading a lot of chaos. Vendors can not go and add 
>> anything in the Linux kernel(just a example, SBI is still OS agnostic) 
>> as well.
> 
> The concept of vendor extensions to a userspace ISA is so fundamentally
> broken to begin with, this FW proposal verges on utter insanity to be
> honest.

Where exactly do you see mentioned that this would be for user space?

> Calling FW for things so deeply ingrained in the kernel such as context
> swithes, isn't going to fly. The performance will be ridiculously bad,
> among other things.

Yes, but I haven't seen anyone propose SBI calls in fast paths?

What I would like to see SBI is used for is:

  * clocks
  * regulators
  * [debug interface (like your debug serial), has the after taste of
BIOS though]
  * power control
  * TEE switch-over

Basically, everything that PSCI and SCMI do on ARM.

Beyond that, there should be very little need for anything in SBI from
what I can tell? Even the IPI mechanism seems ... eh ... odd to me.

Atish, can you please elaborate what exactly you were thinking of as
vendor extensions?

> If Risc-V wants to avoid fragmentation, it should simply keep a firmer
> reign in what is allowed in an implementation and keep a lid on such
> horrid ideas as vendor specific userspace ISA extensions.

I tend to agree here. I think a global identification scheme is
reasonable. But can't we just live with a single global spec that people
contribute to?

That way, people can educate each other on "bad ideas" as well as maybe
see potential for overlapping functionality that multiple vendors may have.

Alex

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