Re: [RFC V1 05/13] vhost-vdpa: VHOST_IOTLB_REMAP

[Jason Wang]

On Thu, Jan 18, 2024 at 4:32 AM Steven Sistare
<steven.sistare@xxxxxxxxxx> wrote:
>
> On 1/10/2024 10:08 PM, Jason Wang wrote:
> > On Thu, Jan 11, 2024 at 4:40 AM Steve Sistare <steven.sistare@xxxxxxxxxx> wrote:
> >>
> >> When device ownership is passed to a new process via VHOST_NEW_OWNER,
> >> some devices need to know the new userland addresses of the dma mappings.
> >> Define the new iotlb message type VHOST_IOTLB_REMAP to update the uaddr
> >> of a mapping.  The new uaddr must address the same memory object as
> >> originally mapped.
> >>
> >> The user must suspend the device before the old address is invalidated,
> >> and cannot resume it until after VHOST_IOTLB_REMAP is called, but this
> >> requirement is not enforced by the API.
> >>
> >> Signed-off-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
> >> ---
> >>  drivers/vhost/vdpa.c             | 34 ++++++++++++++++++++++++++++++++
> >>  include/uapi/linux/vhost_types.h | 11 ++++++++++-
> >>  2 files changed, 44 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> >> index faed6471934a..ec5ca20bd47d 100644
> >> --- a/drivers/vhost/vdpa.c
> >> +++ b/drivers/vhost/vdpa.c
> >> @@ -1219,6 +1219,37 @@ static int vhost_vdpa_pa_map(struct vhost_vdpa *v,
> >>
> >>  }
> >>
> >> +static int vhost_vdpa_process_iotlb_remap(struct vhost_vdpa *v,
> >> +                                         struct vhost_iotlb *iotlb,
> >> +                                         struct vhost_iotlb_msg *msg)
> >> +{
> >> +       struct vdpa_device *vdpa = v->vdpa;
> >> +       const struct vdpa_config_ops *ops = vdpa->config;
> >> +       u32 asid = iotlb_to_asid(iotlb);
> >> +       u64 start = msg->iova;
> >> +       u64 last = start + msg->size - 1;
> >> +       struct vhost_iotlb_map *map;
> >> +       int r = 0;
> >> +
> >> +       if (msg->perm || !msg->size)
> >> +               return -EINVAL;
> >> +
> >> +       map = vhost_iotlb_itree_first(iotlb, start, last);
> >> +       if (!map)
> >> +               return -ENOENT;
> >> +
> >> +       if (map->start != start || map->last != last)
> >> +               return -EINVAL;
> >> +
> >> +       /* batch will finish with remap.  non-batch must do it now. */
> >> +       if (!v->in_batch)
> >> +               r = ops->set_map(vdpa, asid, iotlb);
> >> +       if (!r)
> >> +               map->addr = msg->uaddr;
> >
> > I may miss something, for example for PA mapping,
> >
> > 1) need to convert uaddr into phys addr
> > 2) need to check whether the uaddr is backed by the same page or not?
>
> This code does not verify that the new size@uaddr points to the same physical
> pages as the old size@uaddr.  If the app screws up and they differ, then the app
> may corrupt its own memory, but no-one else's.
>
> It would be expensive for large memories to verify page by page, O(npages), and such
> verification lies on the critical path for virtual machine downtime during live update.
> I could compare the properties of the vma(s) for the old size@uaddr vs the vma for the
> new, but that is more complicated and would be a maintenance headache.  When I submitted
> such code to Alex W when writing the equivalent patches for vfio, he said don't check,
> correctness is the user's responsibility.

Ok, let's document this somewhere.

Thanks

>
> - Steve
>
> >> +
> >> +       return r;
> >> +}
> >> +
> >>  static int vhost_vdpa_process_iotlb_update(struct vhost_vdpa *v,
> >>                                            struct vhost_iotlb *iotlb,
> >>                                            struct vhost_iotlb_msg *msg)
> >> @@ -1298,6 +1329,9 @@ static int vhost_vdpa_process_iotlb_msg(struct vhost_dev *dev, u32 asid,
> >>                         ops->set_map(vdpa, asid, iotlb);
> >>                 v->in_batch = false;
> >>                 break;
> >> +       case VHOST_IOTLB_REMAP:
> >> +               r = vhost_vdpa_process_iotlb_remap(v, iotlb, msg);
> >> +               break;
> >>         default:
> >>                 r = -EINVAL;
> >>                 break;
> >> diff --git a/include/uapi/linux/vhost_types.h b/include/uapi/linux/vhost_types.h
> >> index 9177843951e9..35908315ff55 100644
> >> --- a/include/uapi/linux/vhost_types.h
> >> +++ b/include/uapi/linux/vhost_types.h
> >> @@ -79,7 +79,7 @@ struct vhost_iotlb_msg {
> >>  /*
> >>   * VHOST_IOTLB_BATCH_BEGIN and VHOST_IOTLB_BATCH_END allow modifying
> >>   * multiple mappings in one go: beginning with
> >> - * VHOST_IOTLB_BATCH_BEGIN, followed by any number of
> >> + * VHOST_IOTLB_BATCH_BEGIN, followed by any number of VHOST_IOTLB_REMAP or
> >>   * VHOST_IOTLB_UPDATE messages, and ending with VHOST_IOTLB_BATCH_END.
> >>   * When one of these two values is used as the message type, the rest
> >>   * of the fields in the message are ignored. There's no guarantee that
> >> @@ -87,6 +87,15 @@ struct vhost_iotlb_msg {
> >>   */
> >>  #define VHOST_IOTLB_BATCH_BEGIN    5
> >>  #define VHOST_IOTLB_BATCH_END      6
> >> +
> >> +/*
> >> + * VHOST_IOTLB_REMAP registers a new uaddr for the existing mapping at iova.
> >> + * The new uaddr must address the same memory object as originally mapped.
> >> + * Failure to do so will result in user memory corruption and/or device
> >> + * misbehavior.  iova and size must match the arguments used to create the
> >> + * an existing mapping.  Protection is not changed, and perm must be 0.
> >> + */
> >> +#define VHOST_IOTLB_REMAP          7
> >>         __u8 type;
> >>  };
> >>
> >> --
> >> 2.39.3
> >>
> >
>