Re: [PATCH v6 2/4] vhost-vdpa: introduce SUSPEND backend feature bit

[Eugenio Perez Martin <eperezma@redhat.com>]

On Tue, Jun 28, 2022 at 3:43 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
>
> On Thu, Jun 23, 2022 at 06:07:36PM +0200, Eugenio Pérez wrote:
> >Userland knows if it can suspend the device or not by checking this feature
> >bit.
> >
> >It's only offered if the vdpa driver backend implements the suspend()
> >operation callback, and to offer it or userland to ack it if the backend
> >does not offer that callback is an error.
>
> Should we document in the previous patch that the callback must be
> implemented only if the drive/device support it?
>

It's marked as optional in the doc, following other optional callbacks
like set_group_asid for example. But I'm ok with documenting this
behavior further.

> The rest LGTM although I have a doubt whether it is better to move this
> patch after patch 3, or merge it with patch 3, for bisectability since
> we enable the feature here but if the userspace calls ioctl() with
> VHOST_VDPA_SUSPEND we reply back that it is not supported.
>

I'm fine with moving it, but we will have that behavior with all the
devices anyway. Regarding userspace, we just replace ENOIOCTL with
EOPNOTSUPP. Or I'm missing something?

Thanks!

> Thanks,
> Stefano
>
> >
> >Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> >---
> > drivers/vhost/vdpa.c             | 16 +++++++++++++++-
> > include/uapi/linux/vhost_types.h |  2 ++
> > 2 files changed, 17 insertions(+), 1 deletion(-)
> >
> >diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> >index 23dcbfdfa13b..3d636e192061 100644
> >--- a/drivers/vhost/vdpa.c
> >+++ b/drivers/vhost/vdpa.c
> >@@ -347,6 +347,14 @@ static long vhost_vdpa_set_config(struct vhost_vdpa *v,
> >       return 0;
> > }
> >
> >+static bool vhost_vdpa_can_suspend(const struct vhost_vdpa *v)
> >+{
> >+      struct vdpa_device *vdpa = v->vdpa;
> >+      const struct vdpa_config_ops *ops = vdpa->config;
> >+
> >+      return ops->suspend;
> >+}
> >+
> > static long vhost_vdpa_get_features(struct vhost_vdpa *v, u64 __user *featurep)
> > {
> >       struct vdpa_device *vdpa = v->vdpa;
> >@@ -577,7 +585,11 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
> >       if (cmd == VHOST_SET_BACKEND_FEATURES) {
> >               if (copy_from_user(&features, featurep, sizeof(features)))
> >                       return -EFAULT;
> >-              if (features & ~VHOST_VDPA_BACKEND_FEATURES)
> >+              if (features & ~(VHOST_VDPA_BACKEND_FEATURES |
> >+                               BIT_ULL(VHOST_BACKEND_F_SUSPEND)))
> >+                      return -EOPNOTSUPP;
> >+              if ((features & BIT_ULL(VHOST_BACKEND_F_SUSPEND)) &&
> >+                   !vhost_vdpa_can_suspend(v))
> >                       return -EOPNOTSUPP;
> >               vhost_set_backend_features(&v->vdev, features);
> >               return 0;
> >@@ -628,6 +640,8 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
> >               break;
> >       case VHOST_GET_BACKEND_FEATURES:
> >               features = VHOST_VDPA_BACKEND_FEATURES;
> >+              if (vhost_vdpa_can_suspend(v))
> >+                      features |= BIT_ULL(VHOST_BACKEND_F_SUSPEND);
> >               if (copy_to_user(featurep, &features, sizeof(features)))
> >                       r = -EFAULT;
> >               break;
> >diff --git a/include/uapi/linux/vhost_types.h b/include/uapi/linux/vhost_types.h
> >index 634cee485abb..1bdd6e363f4c 100644
> >--- a/include/uapi/linux/vhost_types.h
> >+++ b/include/uapi/linux/vhost_types.h
> >@@ -161,5 +161,7 @@ struct vhost_vdpa_iova_range {
> >  * message
> >  */
> > #define VHOST_BACKEND_F_IOTLB_ASID  0x3
> >+/* Device can be suspended */
> >+#define VHOST_BACKEND_F_SUSPEND  0x4
> >
> > #endif
> >--
> >2.31.1
> >
>