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Mon, 2 Sep 2024 06:42:34 +0000 From: Shinas Rasheed To: "jonathan.cameron@huawei.com" , "dan.j.williams@intel.com" CC: "Ravis.OpenSrc@micron.com" , "ajayjoshi@micron.com" , "emirakhur@micron.com" , "john@jagalactic.com" , "linux-cxl@vger.kernel.org" , "linux-mm@kvack.org" , "sthanneeru.opensrc@micron.com" , "sthanneeru@micron.com" , Haseeb Gani , Sathesh B Edara , Srinivasa Rao Jampala , Shariful Alam , Vimlesh Kumar , Dharmesh Pitroda Subject: Re: [RFC PATCH] cxl/pci: Set default timeout for background operations Thread-Topic: [RFC PATCH] cxl/pci: Set default timeout for background operations Thread-Index: AQHa/P7j8X+ZQdhQjUCOrX5T/sFRTA== Date: Mon, 2 Sep 2024 06:42:34 +0000 Message-ID: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: msip_labels: x-ms-publictraffictype: Email x-ms-traffictypediagnostic: PH0PR18MB4734:EE_|DM6PR18MB3444:EE_ x-ms-office365-filtering-correlation-id: ea8627da-40a7-42d0-1173-08dccb1a6d63 x-ms-exchange-senderadcheck: 1 x-ms-exchange-antispam-relay: 0 x-microsoft-antispam: BCL:0;ARA:13230040|1800799024|366016|7416014|376014|38070700018; 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I'm sorry if mo= re on the topic has been discussed elsewhere, =0A= but=A0this is a patch that I could find which related to the same topic.=0A= =0A= The requirement for default timeouts (or atleast a mechanism to provide tim= eouts to the command structure) seems to be needed=0A= since the CXL spec doesn't seem to explicitly deny userspace access to back= ground commands. This seems to constrain a lot of=0A= functionality with respect to Vendor Defined commands as well, if a command= needs to be implemented as background.=0A= =0A= >> sthanneeru.opensrc@ wrote:=0A= >> > From: Srinivasulu Thanneeru =0A= >> > =0A= >> > The CXL 3.0 specification outlines background operations,=0A= >> > and support for handling these operations was added in following patch= .=0A= >> > =0A= >> > Link: https://lore.kernel.org/all/20230523170927.20685-5-dave@stgolabs= .net/=0A= >> > =0A= >> > Mailbox commands like =91Log Populate=92 use background operations=0A= >> > to complete the execution of the command.=0A= >> > This can lead to a timeout, since there is currently no option=0A= >> > in the ioctl cxl_send_command structure to specify=0A= >> > a timeout value. The default values being zero can lead=0A= >> > to the driver reporting false negatives to the application.=0A= >> > =0A= >> > This patch aims to establish default values, enabling mailbox commands= =0A= >> > that operate in the background to continue functioning even=0A= >> > if a timeout is not set in the userspace application. =0A= >> =0A= >> The reason there are no defaults is because userspace is not allowed to= =0A= >> issue background commands. The CXL background command definition is=0A= >> awkward in that it allows a single command to monopolize the mailbox for= =0A= >> an indefinite amount of time.=0A= =0A= I understand this is a design adopted by the kernel, and not actually by th= e CXL spec (at least I don't see it=0A= specifying explicitly that background commands are not allowed in band).=0A= =0A= >> =0A= >> Instead, the approach taken with Firmware Update and Sanitize is that a= =0A= >> kernel sysfs ABI mediates access to the mailbox and facilitates bounded = =0A= >> timeslices between command submissions. It effectively allows the kernel= =0A= >> to manage fairness and more importantly preempt userspace if it needs to= =0A= >> issue its own commands.=0A= >> =0A= >> I assume you are only seeing this lack of a default due to building with= =0A= >> CONFIG_CXL_MEM_RAW_COMMANDS=3Dy? If yes, "raw" means "raw" and the kerne= l=0A= >> is mostly taken out of the loop of saving userspace from itself.=0A= =0A= This seems to imply that no vendor defined command can be background comman= ds. Please correct me if=0A= I'm wrong, since Vendor defined commands have to be sent as raw commands, a= nd the absence of default timeouts=0A= indicate that the command will be immediately timed out.=0A= =0A= Also, I understand the point that background commands (since there can only= be one) monopolizes the=0A= mailbox, and hence cannot be exposed to userspace and as I see the backgrou= nd command handling goes synchronously=0A= with the usual command handling in the kernel (ie; foreground command has t= o wait for the background command to complete or timeout)=0A= but doesn't this defeat the purpose of the spec defined ability to run a fo= reground command while a background command is=0A= underway?=0A= =0A= >> =0A= >> All that said, ugh, "Log Populate" has no facility to time bound the=0A= >> population of the log. I do not think it is tenable for Linux to=0A= >> surrender mailbox access for an indefinite uninterruptible amount of=0A= >> time... unless you want to handle "Log Populate" like Sanitize where the= =0A= >> unbounded background operation is tolerated because the device is taken= =0A= >> offline?=0A= >=0A= >It may be pointless to do a component state dump only on an offline device= .=0A= >My assumption is this one is hardware debug only. Patches out of=0A= >tree or behind a really scary sounding config variable perhaps?=0A= >Other than vendor log I don't think populate log applies to the other=0A= >log types yet (they don't mention it anyway!)=0A= >=0A= >Jonathan=0A= =0A= Am I wrong to understand that this design decision is still in the open rig= ht now?=0A= =0A= Another concern was that right now, a WARN_ONCE is issued when a raw comman= d is ensued. Maybe this belongs=0A= on a different thread, but are raw commands to be depreciated (if then how = can Vendor Defined commands be sent?)=0A= =0A=