[Qemu-devel] [PATCH V3 3/5] docs: add pvrdma device documentation.

[Marcel Apfelbaum <marcel@redhat.com>]

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Shamir Rabinovitch <shamir.rabinovitch@oracle.com>
---
 docs/pvrdma.txt | 145 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 145 insertions(+)
 create mode 100644 docs/pvrdma.txt

diff --git a/docs/pvrdma.txt b/docs/pvrdma.txt
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+Paravirtualized RDMA Device (PVRDMA)
+====================================
+
+
+1. Description
+===============
+PVRDMA is the QEMU implementation of VMware's paravirtualized RDMA device.
+It works with its Linux Kernel driver AS IS, no need for any special guest
+modifications.
+
+While it complies with the VMware device, it can also communicate with bare
+metal RDMA-enabled machines and does not require an RDMA HCA in the host, it
+can work with Soft-RoCE (rxe).
+
+It does not require the whole guest RAM to be pinned allowing memory
+over-commit and, even if not implemented yet, migration support will be
+possible with some HW assistance.
+
+A project presentation accompany this document:
+- http://events.linuxfoundation.org/sites/events/files/slides/lpc-2017-pvrdma-marcel-apfelbaum-yuval-shaia.pdf
+
+
+
+2. Setup
+========
+
+
+2.1 Guest setup
+===============
+Fedora 27+ kernels work out of the box, older distributions
+require updating the kernel to 4.14 to include the pvrdma driver.
+
+However the libpvrdma library needed by User Level Software is still
+not available as part of the distributions, so the rdma-core library
+needs to be compiled and optionally installed.
+
+Please follow the instructions at:
+  https://github.com/linux-rdma/rdma-core.git
+
+
+2.2 Host Setup
+==============
+The pvrdma backend is an ibdevice interface that can be exposed
+either by a Soft-RoCE(rxe) device on machines with no RDMA device,
+or an HCA SRIOV function(VF/PF).
+Note that ibdevice interfaces can't be shared between pvrdma devices,
+each one requiring a separate instance (rxe or SRIOV VF).
+
+
+2.2.1 Soft-RoCE backend(rxe)
+===========================
+A stable version of rxe is required, Fedora 27+ or a Linux
+Kernel 4.14+ is preferred.
+
+The rdma_rxe module is part of the Linux Kernel but not loaded by default.
+Install the User Level library (librxe) following the instructions from:
+https://github.com/SoftRoCE/rxe-dev/wiki/rxe-dev:-Home
+
+Associate an ETH interface with rxe by running:
+   rxe_cfg add eth0
+An rxe0 ibdevice interface will be created and can be used as pvrdma backend.
+
+
+2.2.2 RDMA device Virtual Function backend
+==========================================
+Nothing special is required, the pvrdma device can work not only with
+Ethernet Links, but also Infinibands Links.
+All is needed is an ibdevice with an active port, for Mellanox cards
+will be something like mlx5_6 which can be the backend.
+
+
+2.2.3 QEMU setup
+================
+Configure QEMU with --enable-rdma flag, installing
+the required RDMA libraries.
+
+
+3. Usage
+========
+Currently the device is working only with memory backed RAM
+and it must be mark as "shared":
+   -m 1G \
+   -object memory-backend-ram,id=mb1,size=1G,share \
+   -numa node,memdev=mb1 \
+
+The pvrdma device is composed of two functions:
+ - Function 0 is a vmxnet Ethernet Device which is redundant in Guest
+   but is required to pass the ibdevice GID using its MAC.
+   Examples:
+     For an rxe backend using eth0 interface it will use its mac:
+       -device vmxnet3,addr=<slot>.0,multifunction=on,mac=<eth0 MAC>
+     For an SRIOV VF, we take the Ethernet Interface exposed by it:
+       -device vmxnet3,multifunction=on,mac=<RoCE eth MAC>
+ - Function 1 is the actual device:
+       -device pvrdma,addr=<slot>.1,backend-dev=<ibdevice>,backend-gid-idx=<gid>,backend-port=<port>
+   where the ibdevice can be rxe or RDMA VF (e.g. mlx5_4)
+ Note: Pay special attention that the GID at backend-gid-idx matches vmxnet's MAC.
+ The rules of conversion are part of the RoCE spec, but since manual conversion
+ is not required, spotting problems is not hard:
+    Example: GID: fe80:0000:0000:0000:7efe:90ff:fecb:743a
+             MAC: 7c:fe:90:cb:74:3a
+    Note the difference between the first byte of the MAC and the GID.
+
+
+4. Implementation details
+=========================
+The device acts like a proxy between the Guest Driver and the host
+ibdevice interface.
+On configuration path:
+ - For every hardware resource request (PD/QP/CQ/...) the pvrdma will request
+   a resource from the backend interface, maintaining a 1-1 mapping
+   between the guest and host.
+On data path:
+ - Every post_send/receive received from the guest will be converted into
+   a post_send/receive for the backend. The buffers data will not be touched
+   or copied resulting in near bare-metal performance for large enough buffers.
+ - Completions from the backend interface will result in completions for
+   the pvrdma device.
+
+
+
+5. Limitations
+==============
+- The device obviously is limited by the Guest Linux Driver features implementation
+  of the VMware device API.
+- Memory registration mechanism requires mremap for every page in the buffer in order
+  to map it to a contiguous virtual address range. Since this is not the data path
+  it should not matter much.
+- QEMU cannot map guest RAM from a file descriptor if a pvrdma device is attached,
+  so it can't work with huge pages. The limitation will be addressed in the future,
+  however QEMU allocates Gust RAM with MADV_HUGEPAGE so if there are enough huge
+  pages available, QEMU will use them.
+- As previously stated, migration is not supported yet, however with some hardware
+  support can be done.
+
+
+
+6. Performance
+==============
+By design the pvrdma device exits on each post-send/receive, so for small buffers
+the performance is affected; however for medium buffers it will became close to
+bare metal and from 1MB buffers and  up it reaches bare metal performance.
+(tested with 2 VMs, the pvrdma devices connected to 2 VFs of the same device)
+
+All the above assumes no memory registration is done on data path.
-- 
2.13.5