RE: [PATCH v2 6/7] vfio/platform: Create persistent IRQ handlers

[Tian, Kevin]

> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Saturday, March 9, 2024 7:05 AM
> 
> The vfio-platform SET_IRQS ioctl currently allows loopback triggering of
> an interrupt before a signaling eventfd has been configured by the user,
> which thereby allows a NULL pointer dereference.
> 
> Rather than register the IRQ relative to a valid trigger, register all
> IRQs in a disabled state in the device open path.  This allows mask
> operations on the IRQ to nest within the overall enable state governed
> by a valid eventfd signal.  This decouples @masked, protected by the
> @locked spinlock from @trigger, protected via the @igate mutex.
> 
> In doing so, it's guaranteed that changes to @trigger cannot race the
> IRQ handlers because the IRQ handler is synchronously disabled before
> modifying the trigger, and loopback triggering of the IRQ via ioctl is
> safe due to serialization with trigger changes via igate.
> 
> For compatibility, request_irq() failures are maintained to be local to
> the SET_IRQS ioctl rather than a fatal error in the open device path.
> This allows, for example, a userspace driver with polling mode support
> to continue to work regardless of moving the request_irq() call site.
> This necessarily blocks all SET_IRQS access to the failed index.
> 
> Cc: Eric Auger <eric.auger@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 57f972e2b341 ("vfio/platform: trigger an interrupt via eventfd")
> Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>