Re: [PATCH 5/7] cxl/mbox: Allow userspace background commands

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Tue, 17 Jun 2025 12:36:09 -0700
Davidlohr Bueso <dave@stgolabs.net> wrote:

> CXL 3.1 introduced the ability to request that the current on-going
> background command be aborted. As such userspace may be allowed to
> monopolize the background operation capability if there are no
> other bg commands that want to run.
> 
> The policy here are as follows:
> 
> 1. Give bg-capable commands from the kernel higher priority than those
> from userspace (albeit currently no command). Therefore a userspace
> on-going command can be canceled so a kernel one may run instead.
> 
> 2. Userspace bg commands may cancel each other on demand. It is not
> up to the kernel to put any policies in place here.
> 
> 3. Any previously synchronous polling timedout command that is still
> running may also be aborted by any command. This window is expected
> to be small.
> 
> The context of doing the cancellation request is the same as the new
> incoming command, and will always hold the mbox_mutex, guaranteeing
> that any successful cancel does not race with a third thread coming
> in and stealing the effort.
> 
> Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
> ---
>  drivers/cxl/core/mbox.c |  9 ++++--
>  drivers/cxl/cxlmem.h    |  4 +++
>  drivers/cxl/pci.c       | 72 +++++++++++++++++++++++++++++++++++++----
>  include/cxl/mailbox.h   |  2 ++
>  4 files changed, 78 insertions(+), 9 deletions(-)

> diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
> index 3648efcc7c89..6ada0f502914 100644
> --- a/drivers/cxl/pci.c
> +++ b/drivers/cxl/pci.c

> @@ -263,6 +264,8 @@ static int __cxl_pci_mbox_send_cmd(struct cxl_mailbox *cxl_mbox,
>  	 * commands during this window.
>  	 */
>  	switch (mds->bg.opcode) {
> +	case 0:

I assume this is the race?  Add a comment so we can figure this out in future.

> +		break;
>  	case CXL_MBOX_OP_SANITIZE:
>  		/*
>  		 * Ensure no new command comes in until so. Keep the


>  static int cxl_pci_mbox_send(struct cxl_mailbox *cxl_mbox,
>  			     struct cxl_mbox_cmd *cmd)
>  {
> @@ -381,9 +437,11 @@ static int cxl_pci_mbox_send(struct cxl_mailbox *cxl_mbox,
>  	/*
>  	 * Ensure cxl_mbox_background_done() checks are safe amongst
>  	 * each other: no new bg operation can occur in between while polling.
> +	 *

Stray change.

>  	 */