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authorStefan Hajnoczi <stefanha@redhat.com>2020-11-09 17:43:55 +0000
committerMichael S. Tsirkin <mst@redhat.com>2020-11-12 09:19:40 -0500
commit3009edff8192991293fe9e2b50b0d90db83c4a89 (patch)
tree77828bcf3dfae50511ec9cb35728de607e6f2fa7 /docs
parentc6f28ed5075df79fef39c500362a3f4089256c9c (diff)
vhost-user: fix VHOST_USER_ADD/REM_MEM_REG truncation
QEMU currently truncates the mmap_offset field when sending VHOST_USER_ADD_MEM_REG and VHOST_USER_REM_MEM_REG messages. The struct layout looks like this: typedef struct VhostUserMemoryRegion { uint64_t guest_phys_addr; uint64_t memory_size; uint64_t userspace_addr; uint64_t mmap_offset; } VhostUserMemoryRegion; typedef struct VhostUserMemRegMsg { uint32_t padding; /* WARNING: there is a 32-bit hole here! */ VhostUserMemoryRegion region; } VhostUserMemRegMsg; The payload size is calculated as follows when sending the message in hw/virtio/vhost-user.c: msg->hdr.size = sizeof(msg->payload.mem_reg.padding) + sizeof(VhostUserMemoryRegion); This calculation produces an incorrect result of only 36 bytes. sizeof(VhostUserMemRegMsg) is actually 40 bytes. The consequence of this is that the final field, mmap_offset, is truncated. This breaks x86_64 TCG guests on s390 hosts. Other guest/host combinations may get lucky if either of the following holds: 1. The guest memory layout does not need mmap_offset != 0. 2. The host is little-endian and mmap_offset <= 0xffffffff so the truncation has no effect. Fix this by extending the existing 32-bit padding field to 64-bit. Now the padding reflects the actual compiler padding. This can be verified using pahole(1). Also document the layout properly in the vhost-user specification. The vhost-user spec did not document the exact layout. It would be impossible to implement the spec without looking at the QEMU source code. Existing vhost-user frontends and device backends continue to work after this fix has been applied. The only change in the wire protocol is that QEMU now sets hdr.size to 40 instead of 36. If a vhost-user implementation has a hardcoded size check for 36 bytes, then it will fail with new QEMUs. Both QEMU and DPDK/SPDK don't check the exact payload size, so they continue to work. Fixes: f1aeb14b0809e313c74244d838645ed25e85ea63 ("Transmit vhost-user memory regions individually") Cc: Raphael Norwitz <raphael.norwitz@nutanix.com> Cc: Cornelia Huck <cohuck@redhat.com> Cc: Michael S. Tsirkin <mst@redhat.com> Cc: Christian Borntraeger <borntraeger@de.ibm.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Message-Id: <20201109174355.1069147-1-stefanha@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Fixes: f1aeb14b0809 ("Transmit vhost-user memory regions individually") Reviewed-by: Cornelia Huck <cohuck@redhat.com> Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Diffstat (limited to 'docs')
-rw-r--r--docs/interop/vhost-user.rst21
1 files changed, 19 insertions, 2 deletions
diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index 988f154144..6d4025ba6a 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -131,6 +131,23 @@ A region is:
:mmap offset: 64-bit offset where region starts in the mapped memory
+Single memory region description
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
++---------+---------------+------+--------------+-------------+
+| padding | guest address | size | user address | mmap offset |
++---------+---------------+------+--------------+-------------+
+
+:padding: 64-bit
+
+:guest address: a 64-bit guest address of the region
+
+:size: a 64-bit size
+
+:user address: a 64-bit user address
+
+:mmap offset: 64-bit offset where region starts in the mapped memory
+
Log description
^^^^^^^^^^^^^^^
@@ -1281,7 +1298,7 @@ Master message types
``VHOST_USER_ADD_MEM_REG``
:id: 37
:equivalent ioctl: N/A
- :slave payload: memory region
+ :slave payload: single memory region description
When the ``VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS`` protocol
feature has been successfully negotiated, this message is submitted
@@ -1296,7 +1313,7 @@ Master message types
``VHOST_USER_REM_MEM_REG``
:id: 38
:equivalent ioctl: N/A
- :slave payload: memory region
+ :slave payload: single memory region description
When the ``VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS`` protocol
feature has been successfully negotiated, this message is submitted