On Wed, 14 Feb 2024 19:31:23 -0800 Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > sthanneeru.opensrc@ wrote: > > From: Srinivasulu Thanneeru <sthanneeru.opensrc@xxxxxxxxxx> > > > > The CXL 3.0 specification outlines background operations, > > and support for handling these operations was added in following patch. > > > > Link: https://lore.kernel.org/all/20230523170927.20685-5-dave@xxxxxxxxxxxx/ > > > > Mailbox commands like ‘Log Populate’ use background operations > > to complete the execution of the command. > > This can lead to a timeout, since there is currently no option > > in the ioctl cxl_send_command structure to specify > > a timeout value. The default values being zero can lead > > to the driver reporting false negatives to the application. > > > > This patch aims to establish default values, enabling mailbox commands > > that operate in the background to continue functioning even > > if a timeout is not set in the userspace application. > > The reason there are no defaults is because userspace is not allowed to > issue background commands. The CXL background command definition is > awkward in that it allows a single command to monopolize the mailbox for > an indefinite amount of time. > > Instead, the approach taken with Firmware Update and Sanitize is that a > kernel sysfs ABI mediates access to the mailbox and facilitates bounded > timeslices between command submissions. It effectively allows the kernel > to manage fairness and more importantly preempt userspace if it needs to > issue its own commands. > > I assume you are only seeing this lack of a default due to building with > CONFIG_CXL_MEM_RAW_COMMANDS=y? If yes, "raw" means "raw" and the kernel > is mostly taken out of the loop of saving userspace from itself. > > All that said, ugh, "Log Populate" has no facility to time bound the > population of the log. I do not think it is tenable for Linux to > surrender mailbox access for an indefinite uninterruptible amount of > time... unless you want to handle "Log Populate" like Sanitize where the > unbounded background operation is tolerated because the device is taken > offline? It may be pointless to do a component state dump only on an offline device. My assumption is this one is hardware debug only. Patches out of tree or behind a really scary sounding config variable perhaps? Other than vendor log I don't think populate log applies to the other log types yet (they don't mention it anyway!) Jonathan
