Re: [Qemu-devel] [PATCH 6/6] spec/vhost-user spec: Add IOMMU support

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 11, 2017 at 02:32:46PM +0200, Maxime Coquelin wrote:
> This patch specifies and implements the master/slave communication
> to support device IOTLB in slave.
> 
> The vhost_iotlb_msg structure introduced for kernel backends is
> re-used, making the design close between the two backends.
> 
> An exception is the use of the secondary channel to enable the
> slave to send IOTLB miss requests to the master.
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>  docs/specs/vhost-user.txt | 75 +++++++++++++++++++++++++++++++++++++++++++++++
>  hw/virtio/vhost-user.c    | 31 ++++++++++++++++++++
>  2 files changed, 106 insertions(+)
> 
> diff --git a/docs/specs/vhost-user.txt b/docs/specs/vhost-user.txt
> index 5fa7016..4a1f0c3 100644
> --- a/docs/specs/vhost-user.txt
> +++ b/docs/specs/vhost-user.txt
> @@ -97,6 +97,23 @@ Depending on the request type, payload can be:
>     log offset: offset from start of supplied file descriptor
>         where logging starts (i.e. where guest address 0 would be logged)
>  
> + * An IOTLB message
> +   ---------------------------------------------------------
> +   | iova | size | user address | permissions flags | type |
> +   ---------------------------------------------------------
> +
> +   IOVA: a 64-bit guest I/O virtual address
> +   Size: a 64-bit size
> +   User address: a 64-bit user address
> +   Permissions flags: a 8-bit bit field:
> +    - Bit 0: Read access
> +    - Bit 1: Write access
> +   Type: a 8-bit IOTLB message type:
> +    - 1: IOTLB miss
> +    - 2: IOTLB update
> +    - 3: IOTLB invalidate
> +    - 4: IOTLB access fail
> +
>  In QEMU the vhost-user message is implemented with the following struct:
>  
>  typedef struct VhostUserMsg {
> @@ -109,6 +126,7 @@ typedef struct VhostUserMsg {
>          struct vhost_vring_addr addr;
>          VhostUserMemory memory;
>          VhostUserLog log;
> +        struct vhost_iotlb_msg iotlb;
>      };
>  } QEMU_PACKED VhostUserMsg;
>  
> @@ -253,6 +271,31 @@ Once the source has finished migration, rings will be stopped by
>  the source. No further update must be done before rings are
>  restarted.
>  
> +IOMMU support
> +-------------
> +
> +When the VIRTIO_F_IOMMU_PLATFORM feature has been negotiated, the master has
> +to send IOTLB entries update & invalidation by sending VHOST_USER_IOTLB_MSG
> +requests to the slave with a struct vhost_iotlb_msg payload. For update events,
> +the iotlb payload has to be filled with the update message type (2), the I/O
> +virtual address, the size, the user virtual address, and the permissions
> +flags. For invalidation events, the iotlb payload has to be filled with the
> +invalidation message type (3), the I/O virtual address and the size. On
> +success, the slave is expected to reply with a zero payload, non-zero
> +otherwise.
> +
> +When the VHOST_USER_PROTOCOL_F_SLAVE_REQ is supported by the slave, and the
> +master initiated the slave to master communication channel using the
> +VHOST_USER_SET_SLAVE_REQ_FD request, the slave can send IOTLB miss and access
> +failure events by sending VHOST_USER_SLAVE_IOTLB_MSG requests to the master
> +with a struct vhost_iotlb_msg payload. For miss events, the iotlb payload has
> +to be filled with the miss message type (1), the I/O virtual address and the
> +permissions flags. For access failure event, the iotlb payload has to be
> +filled with the access failure message type (4), the I/O virtual address and
> +the permissions flags.

I don't think slave should cache invalid entries. If it does not,
how can it detect access failure as opposed to a miss?

> For synchronization purpose, the slave may rely on the
> +reply-ack feature, so the master may send a reply when operation is completed

What does completed mean in this context?

> +if the reply-ack feature is negotiated and slaves requests a reply.
> +

This is not very clear to me.

So slave sends an access to master. Master finds a pte that
overlaps. What does it send to guest? Initial PTE?
All PTEs to cover the request? Part of the PTE that overlaps
the request?


>  Slave communication
>  -------------------
>  
> @@ -514,6 +557,38 @@ Master message types
>        If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, slave must respond
>        with zero for success, non-zero otherwise.
>  
> + * VHOST_USER_IOTLB_MSG
> +
> +      Id: 22
> +      Equivalent ioctl: N/A (equivalent to VHOST_IOTLB_MSG message type)
> +      Master payload: struct vhost_iotlb_msg
> +      Slave payload: u64
> +
> +      Send IOTLB messages with struct vhost_iotlb_msg as payload.
> +      Master sends such requests to update and invalidate entries in the device
> +      IOTLB. The slave has to acknowledge the request with sending zero as u64
> +      payload for success, non-zero otherwise.
> +      This request should be send only when VIRTIO_F_IOMMU_PLATFORM feature
> +      has been successfully negotiated.
> +
> +Slave message types
> +-------------------
> +
> + * VHOST_USER_SLAVE_IOTLB_MSG
> +
> +      Id: 1
> +      Equivalent ioctl: N/A (equivalent to VHOST_IOTLB_MSG message type)
> +      Slave payload: struct vhost_iotlb_msg
> +      Master payload: N/A
> +
> +      Send IOTLB messages with struct vhost_iotlb_msg as payload.
> +      Slave sends such requests to notify of an IOTLB miss, or an IOTLB
> +      access failure. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated,
> +      and slave set the VHOST_USER_NEED_REPLY flag, master must respond with
> +      zero when operation is successfully completed, or non-zero otherwise.
> +      This request should be send only when VIRTIO_F_IOMMU_PLATFORM feature
> +      has been successfully negotiated.
> +
>  VHOST_USER_PROTOCOL_F_REPLY_ACK:
>  -------------------------------
>  The original vhost-user specification only demands replies for certain
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index fbc09fa..2c93181 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -63,11 +63,13 @@ typedef enum VhostUserRequest {
>      VHOST_USER_SEND_RARP = 19,
>      VHOST_USER_NET_SET_MTU = 20,
>      VHOST_USER_SET_SLAVE_REQ_FD = 21,
> +    VHOST_USER_IOTLB_MSG = 22,
>      VHOST_USER_MAX
>  } VhostUserRequest;
>  
>  typedef enum VhostUserSlaveRequest {
>      VHOST_USER_SLAVE_NONE = 0,
> +    VHOST_USER_SLAVE_IOTLB_MSG = 1,
>      VHOST_USER_SLAVE_MAX
>  }  VhostUserSlaveRequest;
>  
> @@ -105,6 +107,7 @@ typedef struct VhostUserMsg {
>          struct vhost_vring_addr addr;
>          VhostUserMemory memory;
>          VhostUserLog log;
> +        struct vhost_iotlb_msg iotlb;
>      } payload;
>  } QEMU_PACKED VhostUserMsg;
>  
> @@ -611,6 +614,9 @@ static void slave_read(void *opaque)
>      }
>  
>      switch (msg.request) {
> +    case VHOST_USER_SLAVE_IOTLB_MSG:
> +        ret = vhost_backend_handle_iotlb_msg(dev, &msg.payload.iotlb);
> +        break;
>      default:
>          error_report("Received unexpected msg type.");
>          ret = -EINVAL;
> @@ -858,6 +864,29 @@ static int vhost_user_net_set_mtu(struct vhost_dev *dev, uint16_t mtu)
>      return 0;
>  }
>  
> +static int vhost_user_send_device_iotlb_msg(struct vhost_dev *dev,
> +                                            struct vhost_iotlb_msg *imsg)
> +{
> +    VhostUserMsg msg = {
> +        .request = VHOST_USER_IOTLB_MSG,
> +        .size = sizeof(msg.payload.iotlb),
> +        .flags = VHOST_USER_VERSION | VHOST_USER_NEED_REPLY_MASK,
> +        .payload.iotlb = *imsg,
> +    };
> +
> +    if (vhost_user_write(dev, &msg, NULL, 0) < 0) {
> +        return -EFAULT;
> +    }
> +
> +    return process_message_reply(dev, msg.request);
> +}
> +
> +
> +static void vhost_user_set_iotlb_callback(struct vhost_dev *dev, int enabled)
> +{
> +    /* No-op as the receive channel is not dedicated to IOTLB messages. */
> +}
> +
>  const VhostOps user_ops = {
>          .backend_type = VHOST_BACKEND_TYPE_USER,
>          .vhost_backend_init = vhost_user_init,
> @@ -882,4 +911,6 @@ const VhostOps user_ops = {
>          .vhost_migration_done = vhost_user_migration_done,
>          .vhost_backend_can_merge = vhost_user_can_merge,
>          .vhost_net_set_mtu = vhost_user_net_set_mtu,
> +        .vhost_set_iotlb_callback = vhost_user_set_iotlb_callback,
> +        .vhost_send_device_iotlb_msg = vhost_user_send_device_iotlb_msg,
>  };
> -- 
> 2.9.3