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Tue, 25 May 2021 08:01:01 +0000 From: Damien Le Moal To: Greg KH CC: Palmer Dabbelt , "guoren@kernel.org" , Anup Patel , Paul Walmsley , "aou@eecs.berkeley.edu" , "pbonzini@redhat.com" , "corbet@lwn.net" , "graf@amazon.com" , Atish Patra , Alistair Francis , "anup@brainfault.org" , "kvm@vger.kernel.org" , "kvm-riscv@lists.infradead.org" , "linux-riscv@lists.infradead.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-staging@lists.linux.dev" Subject: Re: [PATCH v18 00/18] KVM RISC-V Support Thread-Topic: [PATCH v18 00/18] KVM RISC-V Support Thread-Index: AQHXTGAgKECmO/ER7EehkQtXersO1g== Date: Tue, 25 May 2021 08:01:01 +0000 Message-ID: References: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: authentication-results: linuxfoundation.org; dkim=none (message not signed) header.d=none;linuxfoundation.org; dmarc=none action=none header.from=wdc.com; x-originating-ip: [2400:2411:43c0:6000:9d12:5efd:fc6d:4ecd] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 9355121c-ce25-4381-ae9a-08d91f533ce8 x-ms-traffictypediagnostic: DM6PR04MB5996: x-ms-exchange-transport-forked: True x-microsoft-antispam-prvs: wdcipoutbound: EOP-TRUE x-ms-oob-tlc-oobclassifiers: OLM:10000; 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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: wdc.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: DM6PR04MB7081.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 9355121c-ce25-4381-ae9a-08d91f533ce8 X-MS-Exchange-CrossTenant-originalarrivaltime: 25 May 2021 08:01:01.3706 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: b61c8803-16f3-4c35-9b17-6f65f441df86 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: khNl1E77Z884NdL90degL8CQ/jcQq8lLBf/ptl/jnl1oONyWN3s8nPMdQ394YqUqMIAC7Iof1//bKeEbA0MA9Q== X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR04MB5996 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/05/25 16:37, Greg KH wrote:=0A= > On Mon, May 24, 2021 at 11:08:30PM +0000, Damien Le Moal wrote:=0A= >> On 2021/05/25 7:57, Palmer Dabbelt wrote:=0A= >>> On Mon, 24 May 2021 00:09:45 PDT (-0700), guoren@kernel.org wrote:=0A= >>>> Thx Anup,=0A= >>>>=0A= >>>> Tested-by: Guo Ren (Just on qemu-rv64)=0A= >>>>=0A= >>>> I'm following your KVM patchset and it's a great job for riscv=0A= >>>> H-extension. I think hardware companies hope Linux KVM ready first=0A= >>>> before the real chip. That means we can ensure the hardware could run= =0A= >>>> mainline linux.=0A= >>>=0A= >>> I understand that it would be wonderful for hardware vendors to have a = =0A= >>> guarantee that their hardware will be supported by the software =0A= >>> ecosystem, but that's not what we're talking about here. Specifically,= =0A= >>> the proposal for this code is to track the latest draft extension which= =0A= >>> would specifically leave vendors who implement the current draft out in= =0A= >>> the cold was something to change. In practice that is the only way to = =0A= >>> move forward with any draft extension that doesn't have hardware =0A= >>> available, as the software RISC-V implementations rapidly deprecate =0A= >>> draft extensions and without a way to test our code it is destined to = =0A= >>> bit rot.=0A= >>=0A= >> To facilitate the process of implementing, and updating, against draft= =0A= >> specifications, I proposed to have arch/riscv/staging added. This would = be the=0A= >> place to put code based on drafts. Some simple rules can be put in place= :=0A= >> 1) The code and eventual ABI may change any time, no guarantees of backw= ard=0A= >> compatibility=0A= >> 2) Once the specifications are frozen, the code is moved out of staging= =0A= >> somewhere else.=0A= >> 3) The code may be removed any time if the specification proposal is dro= pped, or=0A= >> any other valid reason (can't think of any other right now)=0A= >> 4) ...=0A= >>=0A= >> This way, the implementation process would be greatly facilitated and=0A= >> interactions between different extensions can be explored much more easi= ly.=0A= >>=0A= >> Thoughts ?=0A= > =0A= > It will not work, unless you are mean and ruthless and people will get=0A= > mad at you. I do not recommend it at all.=0A= > =0A= > Once code shows up in the kernel tree, and people rely on it, you now=0A= > _have_ to support it. Users don't know the difference between "staging= =0A= > or not staging" at all. We have reported problems of staging media=0A= > drivers breaking userspace apps and people having problems with that,=0A= > despite the media developers trying to tell the world, "DO NOT RELY ON=0A= > THESE!".=0A= > =0A= > And if this can't be done with tiny simple single drivers, you are going= =0A= > to have a world-of-hurt if you put arch/platform support into=0A= > arch/riscv/. Once it's there, you will never be able to delete it,=0A= > trust me.=0A= =0A= All very good points. Thank you for sharing.=0A= =0A= > If you REALLY wanted to do this, you could create drivers/staging/riscv/= =0A= > and try to make the following rules:=0A= > =0A= > - stand-alone code only, can not depend on ANYTHING outside of=0A= > the directory that is not also used by other in-kernel code=0A= > - does not expose any userspace apis=0A= > - interacts only with existing in-kernel code.=0A= > - can be deleted at any time, UNLESS someone is using it for=0A= > functionality on a system=0A= > =0A= > But what use would that be? What could you put into there that anyone=0A= > would be able to actually use?=0A= =0A= Yes, you already mentioned this and we were not thinking about this solutio= n.=0A= drivers/staging really is for device drivers and does not apply to arch cod= e.=0A= =0A= > Remember the rule we made to our user community over 15 years ago:=0A= > =0A= > We will not break userspace functionality*=0A= > =0A= > With the caveat of "* - in a way that you notice".=0A= > =0A= > That means we can remove and change things that no one relies on=0A= > anymore, as long as if someone pops up that does rely on it, we put it=0A= > back.=0A= > =0A= > We do this because we never want anyone to be afraid to drop in a new=0A= > kernel, because they know we did not break their existing hardware and=0A= > userspace workloads. And if we did, we will work quickly to fix it.=0A= =0A= Yes, I am well aware of this rule.=0A= =0A= > So back to the original issue here, what is the problem that you are=0A= > trying to solve? Why do you want to have in-kernel code for hardware=0A= > that no one else can have access to, and that isn't part of a "finalized= =0A= > spec" that ends up touching other subsystems and is not self-contained?= =0A= =0A= For the case at hand, the only thing that would be outside of the staging a= rea=0A= would be the ABI definition, but that one depends only on the ratified risc= v ISA=0A= specs. So having it outside of staging would be OK. The idea of the arch st= aging=0A= area is 2 fold:=0A= 1) facilitate the development work overall, both for Paolo and Anup on the = KVM=0A= part, but also others to check that their changes do not break KVM support.= =0A= 2) Provide feedback to the specs groups that their concerns are moot. E.g. = one=0A= reason the hypervisor specs are being delayed is concerns with interrupt=0A= handling. With a working implementation based on current ratified specs for= =0A= other components (e.g. interrupt controller), the hope is that the specs gr= oup=0A= can speed up freezing of the specs.=0A= =0A= But your points about how users will likely end up using this potentially= =0A= creates a lot more problems than we are solving...=0A= =0A= > Why not take the energy here and go get that spec ratified so we aren't= =0A= > having this argument anymore? What needs to be done to make that happen= =0A= > and why hasn't anyone done that? There's nothing keeping kernel=0A= > developers from working on spec groups, right?=0A= =0A= We are participating and giving arguments for freezing the specs. This is= =0A= however not working as we would like. But that is a problem to be addressed= with=0A= RISCV International and the processes governing the operation of specificat= ion=0A= groups. The linux mailing lists are not the right place to discuss this, so= I=0A= will not go into more details.=0A= =0A= Thank you for the feedback.=0A= =0A= Best regards.=0A= =0A= -- =0A= Damien Le Moal=0A= Western Digital Research=0A=