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MMAP.md

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MMAP

The mmap modules are used for fast memory transfers between client and server when both reside on the same host.

Implementations

Component Link
client xpra.client.mixins.mmap
client connection xpra.server.source.mmap
server xpra.server.mixins.mmap

Capabilities

The client and server should expose the following capabilities in their hello packet using the clipboard prefix.

The client creates an mmap backing file, writes a random token at a random position within this mmap area and sends the following capabilities:

Capability Value
file path to the mmap backing file
size size of the mmap area
token random token value generated
token_index position where the token was written
token_bytes length of the token in bytes

The server should attempt to open the mmap file specified, and verify that the token is found.

To use this mmap file, it must write a new token and return this information to the client. (using the same format, excluding the file and size that the client has already specified)

The client then verifies that the mmap file can be used bi-directionally.

Network Packets

There are no specific mmap packets used, mmap is used as an encoding.

virtio-shmem

Xpra can use mmap with virtio-shmem to speed up connections between a host and guest or even between two guests.

Example steps for host to guest setup:

  • add a shmem device to your VM, ie: 
    DEV_NAME="shmem-xpra"
virt-install --shmem name="${DEV_NAME}",model.type=ivshmem-plain,size.unit=M,size=512"`)
  • enable the shared memory device on the guest: 
    echo 1 > $(find /sys/devices/ -type f -name "resource2_wc" -exec dirname "{}" \;)/enable
    and start an xpra server with mmap pointing to it: 
    VSOCK_PORT=10000
MMAP_PATH=$(find /sys/devices/ -type f -name "resource2_wc")
xpra start --bind-vsock=auto:${VSOCK_PORT} --mmap=$MMAP_PATH
  • from the host, use mmap with the same device: 
    DEV_NAME="shmem-xpra"
VSOCK_PORT=10000
VM_CID=$(virsh dumpxml "${DEV_NAME}" | grep cid | sed 's/[^0-9]*//g')
xpra attach vsock://"${VM_CID}":${VSOCK_PORT}/ -d mmap --mmap="/dev/shm/${DEV_NAME}"

Documentation: