Re: [PATCH] vpci: introduce per-domain lock to protect vpci structure

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Feb 14, 2022 at 02:00:26PM +0000, Oleksandr Andrushchenko wrote:
> /*
> * FIXME: apply_map is called from dom0 specific init code when
> * system_state < SYS_STATE_active, so there is no race condition
> * possible between this code and vpci_process_pending. So, neither
> * vpci_process_pending may try to acquire the lock in read mode and
> * also destroy pdev->vpci in its error path nor pdev may be disposed yet.
> * This means that it is not required to check if the relevant pdev
> * still exists after re-acquiring the lock.

I'm not sure why you need to mention vpci_process_pending here:
apply_map and defer_map are mutually exclusive, so given the current
code it's impossible to get in a situation where apply_map is called
while there's pending work on the vCPU (ie: v->vpci.mem != NULL).

Also there's no need for a FIXME tag: the current approach doesn't
require any fixes unless we start using apply_map in a different
context.

Hence I think the comment should be along the lines of:

/*
 * It's safe to drop and reacquire the lock in this context without
 * risking pdev disappearing because devices cannot be removed until the
 * initial domain has been started.
 */

Thanks, Roger.