hw/cxl: Support aborting background commands

hw/cxl: Support aborting background commands

As of 3.1 spec, background commands can be canceled with a new
abort command. Implement the support, which is advertised in
the CEL. No ad-hoc context undoing is necessary as all the
command logic of the running bg command is done upon completion.
Arbitrarily, the on-going background cmd will not be aborted if
already at least 85% done;

A mutex is introduced to stabilize mbox request cancel command vs
the timer callback being fired scenarios (as well as reading the
mbox registers). While some operations under critical regions
may be unnecessary (irq notifying, cmd callbacks), this is not
a path where performance is important, so simplicity is preferred.

Tested-by: Ajay Joshi <ajay.opensrc@micron.com>
Reviewed-by: Ajay Joshi <ajay.opensrc@micron.com>
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20250305092501.191929-2-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>