[Qemu-devel] [RFC 2/2] spec/vhost-user spec: Add IOMMU support

[Maxime Coquelin <maxime.coquelin@redhat.com>]

This patch specifies the master/slave communication to support
device IOTLB implementation 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 | 56 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/docs/specs/vhost-user.txt b/docs/specs/vhost-user.txt
index b365047..048a4d6 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;
 
@@ -258,6 +276,30 @@ 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
+update 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_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. On success, the master is expected to reply  when the
+request has been handled (for example, on miss requests, once the device IOTLB
+has been updated) with a zero payload, non-zero otherwise.
+
 Protocol features
 -----------------
 
@@ -524,6 +566,20 @@ Message types
       has been negotiated, and protocol feature bit VHOST_USER_PROTOCOL_F_SLAVE_REQ
       bit is present in VHOST_USER_GET_PROTOCOL_FEATURES.
 
+ * VHOST_USER_IOTLB_MSG
+
+      Id: 22
+      Equivalent ioctl: N/A (equivalent to VHOST_IOTLB_MSG message type)
+      Initiator: Master or slave
+
+      Send IOTLB messages with struct vhost_iotlb_msg as payload.
+      Master sends such requests to update and invalidate entries in the device
+      IOTLB. Slave sends such requests to notify of an IOTLB miss, or an IOTLB
+      access failure. The recipient 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.
+
 VHOST_USER_PROTOCOL_F_REPLY_ACK:
 -------------------------------
 The original vhost-user specification only demands replies for certain
-- 
2.9.3