Re: [PATCH v1] s390/vfio-ap: Signal eventfd when guest AP configuration is changed

Alex Williamson
2025-01-14T20:05:59Z

On Tue, 7 Jan 2025 13:36:45 -0500
Rorie Reyes <rreyes@linux.ibm.com> wrote:

> In this patch, an eventfd object is created by the vfio_ap device driver
> and used to notify userspace when a guests's AP configuration is
> dynamically changed. Such changes may occur whenever:
>
> * An adapter, domain or control domain is assigned to or unassigned from a
> mediated device that is attached to the guest.
> * A queue assigned to the mediated device that is attached to a guest is
> bound to or unbound from the vfio_ap device driver. This can occur
> either by manually binding/unbinding the queue via the vfio_ap driver's
> sysfs bind/unbind attribute interfaces, or because an adapter, domain or
> control domain assigned to the mediated device is added to or removed
> from the host's AP configuration via an SE/HMC
>
> The purpose of this patch is to provide immediate notification of changes
> made to a guest's AP configuration by the vfio_ap driver. This will enable
> the guest to take immediate action rather than relying on polling or some
> other inefficient mechanism to detect changes to its AP configuration.
>
> Note that there are corresponding QEMU patches that will be shipped along
> with this patch (see vfio-ap: Report vfio-ap configuration changes) that
> will pick up the eventfd signal.
>
> Signed-off-by: Rorie Reyes <rreyes@linux.ibm.com>
> Reviewed-by: Anthony Krowiak <akrowiak@linux.ibm.com>
> Tested-by: Anthony Krowiak <akrowiak@linux.ibm.com>
> ---
> drivers/s390/crypto/vfio_ap_ops.c | 52 ++++++++++++++++++++++++++-
> drivers/s390/crypto/vfio_ap_private.h | 2 ++
> include/uapi/linux/vfio.h | 1 +
> 3 files changed, 54 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> index a52c2690933f..c6ff4ab13f16 100644
> --- a/drivers/s390/crypto/vfio_ap_ops.c
> +++ b/drivers/s390/crypto/vfio_ap_ops.c
> @@ -650,13 +650,22 @@ static void vfio_ap_matrix_init(struct ap_config_info *info,
> matrix->adm_max = info->apxa ? info->nd : 15;
> }
>
> +static void signal_guest_ap_cfg_changed(struct ap_matrix_mdev *matrix_mdev)
> +{
> + if (matrix_mdev->cfg_chg_trigger)
> + eventfd_signal(matrix_mdev->cfg_chg_trigger);
> +}
> +
> static void vfio_ap_mdev_update_guest_apcb(struct ap_matrix_mdev *matrix_mdev)
> {
> - if (matrix_mdev->kvm)
> + if (matrix_mdev->kvm) {
> kvm_arch_crypto_set_masks(matrix_mdev->kvm,
> matrix_mdev->shadow_apcb.apm,
> matrix_mdev->shadow_apcb.aqm,
> matrix_mdev->shadow_apcb.adm);
> +
> + signal_guest_ap_cfg_changed(matrix_mdev);
> + }
> }
>
> static bool vfio_ap_mdev_filter_cdoms(struct ap_matrix_mdev *matrix_mdev)
> @@ -792,6 +801,7 @@ static int vfio_ap_mdev_probe(struct mdev_device *mdev)
> if (ret)
> goto err_put_vdev;
> matrix_mdev->req_trigger = NULL;
> + matrix_mdev->cfg_chg_trigger = NULL;
> dev_set_drvdata(&mdev->dev, matrix_mdev);
> mutex_lock(&matrix_dev->mdevs_lock);
> list_add(&matrix_mdev->node, &matrix_dev->mdev_list);
> @@ -1860,6 +1870,7 @@ static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
> get_update_locks_for_kvm(kvm);
>
> kvm_arch_crypto_clear_masks(kvm);
> + signal_guest_ap_cfg_changed(matrix_mdev);
> vfio_ap_mdev_reset_queues(matrix_mdev);
> kvm_put_kvm(kvm);
> matrix_mdev->kvm = NULL;
> @@ -2097,6 +2108,10 @@ static ssize_t vfio_ap_get_irq_info(unsigned long arg)
> info.count = 1;
> info.flags = VFIO_IRQ_INFO_EVENTFD;
> break;
> + case VFIO_AP_CFG_CHG_IRQ_INDEX:
> + info.count = 1;
> + info.flags = VFIO_IRQ_INFO_EVENTFD;
> + break;
> default:
> return -EINVAL;
> }
> @@ -2160,6 +2175,39 @@ static int vfio_ap_set_request_irq(struct ap_matrix_mdev *matrix_mdev,
> return 0;
> }
>
> +static int vfio_ap_set_cfg_change_irq(struct ap_matrix_mdev *matrix_mdev, unsigned long arg)
> +{
> + s32 fd;
> + void __user *data;
> + unsigned long minsz;
> + struct eventfd_ctx *cfg_chg_trigger;
> +
> + minsz = offsetofend(struct vfio_irq_set, count);
> + data = (void __user *)(arg + minsz);
> +
> + if (get_user(fd, (s32 __user *)data))
> + return -EFAULT;
> +
> + if (fd == -1) {
> + if (matrix_mdev->cfg_chg_trigger)
> + eventfd_ctx_put(matrix_mdev->cfg_chg_trigger);
> + matrix_mdev->cfg_chg_trigger = NULL;
> + } else if (fd >= 0) {
> + cfg_chg_trigger = eventfd_ctx_fdget(fd);
> + if (IS_ERR(cfg_chg_trigger))
> + return PTR_ERR(cfg_chg_trigger);
> +
> + if (matrix_mdev->cfg_chg_trigger)
> + eventfd_ctx_put(matrix_mdev->cfg_chg_trigger);
> +
> + matrix_mdev->cfg_chg_trigger = cfg_chg_trigger;
> + } else {
> + return -EINVAL;
> + }
> +
> + return 0;
> +}

How does this guard against a use after free, such as the eventfd being
disabled or swapped concurrent to a config change? Thanks,

Alex

> +
> static int vfio_ap_set_irqs(struct ap_matrix_mdev *matrix_mdev,
> unsigned long arg)
> {
> @@ -2175,6 +2223,8 @@ static int vfio_ap_set_irqs(struct ap_matrix_mdev *matrix_mdev,
> switch (irq_set.index) {
> case VFIO_AP_REQ_IRQ_INDEX:
> return vfio_ap_set_request_irq(matrix_mdev, arg);
> + case VFIO_AP_CFG_CHG_IRQ_INDEX:
> + return vfio_ap_set_cfg_change_irq(matrix_mdev, arg);
> default:
> return -EINVAL;
> }
> diff --git a/drivers/s390/crypto/vfio_ap_private.h b/drivers/s390/crypto/vfio_ap_private.h
> index 437a161c8659..37de9c69b6eb 100644
> --- a/drivers/s390/crypto/vfio_ap_private.h
> +++ b/drivers/s390/crypto/vfio_ap_private.h
> @@ -105,6 +105,7 @@ struct ap_queue_table {
> * @mdev: the mediated device
> * @qtable: table of queues (struct vfio_ap_queue) assigned to the mdev
> * @req_trigger eventfd ctx for signaling userspace to return a device
> + * @cfg_chg_trigger eventfd ctx to signal AP config changed to userspace
> * @apm_add: bitmap of APIDs added to the host's AP configuration
> * @aqm_add: bitmap of APQIs added to the host's AP configuration
> * @adm_add: bitmap of control domain numbers added to the host's AP
> @@ -120,6 +121,7 @@ struct ap_matrix_mdev {
> struct mdev_device *mdev;
> struct ap_queue_table qtable;
> struct eventfd_ctx *req_trigger;
> + struct eventfd_ctx *cfg_chg_trigger;
> DECLARE_BITMAP(apm_add, AP_DEVICES);
> DECLARE_BITMAP(aqm_add, AP_DOMAINS);
> DECLARE_BITMAP(adm_add, AP_DOMAINS);
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index c8dbf8219c4f..a2d3e1ac6239 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -671,6 +671,7 @@ enum {
> */
> enum {
> VFIO_AP_REQ_IRQ_INDEX,
> + VFIO_AP_CFG_CHG_IRQ_INDEX,
> VFIO_AP_NUM_IRQS
> };
>