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doc/vhost_processing: outline the differences between virtio and vhost

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Change-Id: I205df3cb852e34e8727bbd2e6c9a3a1125c86e50
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/420800
Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
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Dariusz Stojaczyk 2018-07-30 19:41:16 +02:00 committed by Jim Harris
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doc/vhost_processing.md
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@ -6,22 +6,43 @@
- @ref vhost_processing_qemu
- @ref vhost_processing_init
- @ref vhost_processing_io_path
- @ref vhost_spdk_optimizations
# Introduction {#vhost_processing_intro}
This document is intended to provide an overall high level insight into how
Vhost works behind the scenes. It assumes you're already familiar with the
basics of virtqueues and vrings from the
[VIRTIO protocol](http://docs.oasis-open.org/virtio/virtio/v1.0/virtio-v1.0.html).
Vhost works behind the scenes.
Code snippets used in this document might have been simplified for the sake
of readability and should not be used as an API or implementation reference.
vhost is a protocol for devices accessible via inter-process communication.
It uses the same virtqueue and vring layout for I/O transport as VIRTIO to
allow direct mapping to Virtio devices. The initial vhost implementation is
a part of the Linux kernel and uses ioctl interface to communicate with
userspace applications. What makes it possible for SPDK to expose a vhost
device is Vhost-user protocol.
Reading from the
[Virtio specification](http://docs.oasis-open.org/virtio/virtio/v1.0/virtio-v1.0.html):
```
The purpose of virtio and [virtio] specification is that virtual environments
and guests should have a straightforward, efficient, standard and extensible
mechanism for virtual devices, rather than boutique per-environment or per-OS
mechanisms.
```
Virtio devices use virtqueues to transport data efficiently. Virtqueue is a set
of three different single-producer, single-consumer ring structures designed to
store generic scatter-gatter I/O. Virtio is most commonly used in QEMU VMs,
where the QEMU itself exposes a virtual PCI device and the guest OS communicates
with it using a specific Virtio PCI driver. With only Virtio involved, it's
always the QEMU process that handles all I/O traffic.
Vhost is a protocol for devices accessible via inter-process communication.
It uses the same virtqueue layout as Virtio to allow Vhost devices to be mapped
directly to Virtio devices. This allows a Vhost device, exposed by an SPDK
application, to be accessed directly by a guest OS inside a QEMU process with
an existing Virtio (PCI) driver. Only the configuration, I/O submission
notification, and I/O completion interruption are piped through QEMU.
See also @ref vhost_spdk_optimizations
The initial vhost implementation is a part of the Linux kernel and uses ioctl
interface to communicate with userspace applications. What makes it possible for
SPDK to expose a vhost device is Vhost-user protocol.
The [Vhost-user specification](https://git.qemu.org/?p=qemu.git;a=blob_plain;f=docs/interop/vhost-user.txt;hb=HEAD)
describes the protocol as follows:
@ -55,24 +76,19 @@ communicates with SPDK Vhost-SCSI device.
![QEMU/SPDK vhost data flow](img/qemu_vhost_data_flow.svg)
The irqfd mechanism isn't described in this document, as it KVM/QEMU-specific.
Briefly speaking, doing an eventfd_write on the callfd descriptor will
directly interrupt the guest because of irqfd.
# Device initialization {#vhost_processing_init}
All initialization and management information is exchanged via the Vhost-user
All initialization and management information is exchanged using Vhost-user
messages. The connection always starts with the feature negotiation. Both
the Master and the Slave exposes a list of their implemented features. Most
of these features are implementation-related, but also regard e.g. multiqueue
support or live migration. A feature will be used only if both sides support
it.
the Master and the Slave exposes a list of their implemented features and
upon negotiation they choose a common set of those. Most of these features are
implementation-related, but also regard e.g. multiqueue support or live migration.
After the negotiatiation Vhost-user driver shares its memory, so that the vhost
After the negotiatiation, the Vhost-user driver shares its memory, so that the vhost
device (SPDK) can access it directly. The memory can be fragmented into multiple
physically-discontiguous regions, although Vhost-user specification enforces
a limit on their number (currently 8). The driver sends a single message with
the following data for each region:
physically-discontiguous regions and Vhost-user specification puts a limit on
their number - currently 8. The driver sends a single message for each region with
the following data:
* file descriptor - for mmap
* user address - for memory translations in Vhost-user messages (e.g.
translating vring addresses)
@ -85,7 +101,7 @@ The Master will send new memory regions after each memory change - usually
hotplug/hotremove. The previous mappings will be removed.
Drivers may also request a device config, consisting of e.g. disk geometry.
Vhost-SCSI drivers, however, don't need implement this functionality
Vhost-SCSI drivers, however, don't need to implement this functionality
as they use common SCSI I/O to inquiry the underlying disk(s).
Afterwards, the driver requests the number of maximum supported queues and
@ -127,10 +143,9 @@ struct virtio_scsi_req_cmd {
```
Virtqueue generally consists of an array of descriptors and each I/O needs
to be converted into a chain of such descriptors. A descriptor can be
either readable or writable, so each I/O request must consist of at least two
descriptors (request + response).
to be converted into a chain of such descriptors. A single descriptor can be
either readable or writable, so each I/O request consists of at least two
(request + response).
```
struct virtq_desc {
@ -150,6 +165,10 @@ struct virtq_desc {
};
```
Legacy Virtio implementations used the name vring alongside virtqueue, and the
name vring is still used in virtio data structures inside the code. Instead of
`struct virtq_desc`, the `struct vring_desc` is much more likely to be found.
The device after polling this descriptor chain needs to translate and transform
it back into the original request struct. It needs to know the request layout
up-front, so each device backend (Vhost-Block/SCSI) has its own implementation
@ -158,9 +177,19 @@ the Vhost-user memory region table and goes through a gpa_to_vva translation
(guest physical address to vhost virtual address). SPDK enforces the request
and response data to be contained within a single memory region. I/O buffers
do not have such limitations and SPDK may automatically perform additional
iovec splitting and gpa_to_vva translations if required. After forming request
iovec splitting and gpa_to_vva translations if required. After forming the request
structs, SPDK forwards such I/O to the underlying drive and polls for the
completion. Once I/O completes, SPDK vhost fills the response buffer with
proper data and interrupts the guest by doing an eventfd_write on the call
descriptor for proper virtqueue. There are multiple interrupt coalescing
features involved, but they won't be discussed in this document.
features involved, but they are not be discussed in this document.
## SPDK optimizations {#vhost_spdk_optimizations}
Due to its poll-mode nature, SPDK vhost removes the requirement for I/O submission
notifications, drastically increasing the vhost server throughput and decreasing
the guest overhead of submitting an I/O. A couple of different solutions exist
to mitigate the I/O completion interrupt overhead (irqfd, vDPA), but those won't
be discussed in this document. For the highest performance, a poll-mode @ref virtio
can be used, as it suppresses all I/O completion interrupts, making the I/O
path to fully bypass the QEMU/KVM overhead.