Re: [PATCH v11 5/9] s390/pci: Update the logic for detecting passthrough device

[Alex Williamson <alex@shazbot.org>]

On Mon, 16 Mar 2026 12:15:40 -0700
Farhan Ali <alifm@linux.ibm.com> wrote:

> We can now have userspace drivers (vfio-pci based) on s390x. The userspace
> drivers will not have any KVM fd and so no kzdev associated with them. So
> we need to update the logic for detecting passthrough devices to not depend
> on struct kvm_zdev.
> 
> Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
> ---
>  arch/s390/include/asm/pci.h      |  1 +
>  arch/s390/pci/pci_event.c        | 14 ++++----------
>  drivers/vfio/pci/vfio_pci_zdev.c |  9 ++++++++-
>  3 files changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/s390/include/asm/pci.h b/arch/s390/include/asm/pci.h
> index c0ff19dab580..ec8a772bf526 100644
> --- a/arch/s390/include/asm/pci.h
> +++ b/arch/s390/include/asm/pci.h
> @@ -171,6 +171,7 @@ struct zpci_dev {
>  
>  	char res_name[16];
>  	bool mio_capable;
> +	bool mediated_recovery;
>  	struct zpci_bar_struct bars[PCI_STD_NUM_BARS];
>  
>  	u64		start_dma;	/* Start of available DMA addresses */
> diff --git a/arch/s390/pci/pci_event.c b/arch/s390/pci/pci_event.c
> index 839bd91c056e..de504925f709 100644
> --- a/arch/s390/pci/pci_event.c
> +++ b/arch/s390/pci/pci_event.c
> @@ -60,16 +60,10 @@ static inline bool ers_result_indicates_abort(pci_ers_result_t ers_res)
>  	}
>  }
>  
> -static bool is_passed_through(struct pci_dev *pdev)
> +static bool needs_mediated_recovery(struct pci_dev *pdev)
>  {
>  	struct zpci_dev *zdev = to_zpci(pdev);
> -	bool ret;
> -
> -	mutex_lock(&zdev->kzdev_lock);
> -	ret = !!zdev->kzdev;
> -	mutex_unlock(&zdev->kzdev_lock);
> -
> -	return ret;
> +	return zdev->mediated_recovery;

Looks like this was always a bit racy, it's not clear the mutex
provided anything except maybe memory ordering.  Maybe the expectation
is that errors don't occur in relation to open/close_device?  I wonder
if READ/WRITE_ONCE would at least provide some ordering assurances.
Otherwise:

Acked-by: Alex Williamson <alex@shazbot.org>

>  }
>  
>  static bool is_driver_supported(struct pci_driver *driver)
> @@ -194,7 +188,7 @@ static pci_ers_result_t zpci_event_attempt_error_recovery(struct pci_dev *pdev)
>  	}
>  	pdev->error_state = pci_channel_io_frozen;
>  
> -	if (is_passed_through(pdev)) {
> +	if (needs_mediated_recovery(pdev)) {
>  		pr_info("%s: Cannot be recovered in the host because it is a pass-through device\n",
>  			pci_name(pdev));
>  		status_str = "failed (pass-through)";
> @@ -279,7 +273,7 @@ static void zpci_event_io_failure(struct pci_dev *pdev, pci_channel_state_t es)
>  	 * we will inject the error event and let the guest recover the device
>  	 * itself.
>  	 */
> -	if (is_passed_through(pdev))
> +	if (needs_mediated_recovery(pdev))
>  		goto out;
>  	driver = to_pci_driver(pdev->dev.driver);
>  	if (driver && driver->err_handler && driver->err_handler->error_detected)
> diff --git a/drivers/vfio/pci/vfio_pci_zdev.c b/drivers/vfio/pci/vfio_pci_zdev.c
> index 0990fdb146b7..a7bc23ce8483 100644
> --- a/drivers/vfio/pci/vfio_pci_zdev.c
> +++ b/drivers/vfio/pci/vfio_pci_zdev.c
> @@ -148,6 +148,8 @@ int vfio_pci_zdev_open_device(struct vfio_pci_core_device *vdev)
>  	if (!zdev)
>  		return -ENODEV;
>  
> +	zdev->mediated_recovery = true;
> +
>  	if (!vdev->vdev.kvm)
>  		return 0;
>  
> @@ -161,7 +163,12 @@ void vfio_pci_zdev_close_device(struct vfio_pci_core_device *vdev)
>  {
>  	struct zpci_dev *zdev = to_zpci(vdev->pdev);
>  
> -	if (!zdev || !vdev->vdev.kvm)
> +	if (!zdev)
> +		return;
> +
> +	zdev->mediated_recovery = false;
> +
> +	if (!vdev->vdev.kvm)
>  		return;
>  
>  	if (zpci_kvm_hook.kvm_unregister)