Re: [RFC PATCH] cxl/pci: Set default timeout for background operations

Dan Williams
2024-02-15T03:32:18Z

sthanneeru.opensrc@ wrote:
> From: Srinivasulu Thanneeru <sthanneeru.opensrc@micron.com>
>
> 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@stgolabs.net/
>
> 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?