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doc/vhost: add SVG diagram

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Replaced basic ASCII diagram with the complex SVG one.
Also removed slightly misleading vhost introduction
and replaced it with a doc/vhost_processing.md
reference, which does a better job at explaining vhost.

Vhost is a protocol intended to expose Virtio queues to
an external process. While Virtio queues are defined
by the Virtio protocol, it is still misleading to say
vhost devices use Virtio protocol to communicate. They
use Vhost-user protocol. Virtio shouldn't be mentioned
unless we talk about QEMU or guest VM.

Change-Id: I38ce4d3cc9e1c96339d9fa63b5e94019a17ccf9e
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/414816
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
This commit is contained in:
Dariusz Stojaczyk 2018-06-12 18:09:30 +02:00 committed by Jim Harris
parent 6a5ae72b47
commit c06345bfaf
1 changed files with 11 additions and 33 deletions
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44
doc/vhost.md
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@ -15,40 +15,18 @@
A vhost target provides a local storage service as a process running on a local machine.
It is capable of exposing virtualized block devices to QEMU instances or other arbitrary
processes. These processes communicate with the vhost target using the
[virtio protocol](https://wiki.libvirt.org/page/Virtio), a standardized protocol for
paravirtualized devices.
processes.
The following diagram presents how QEMU-based VM communicates with SPDK Vhost-SCSI device.
![QEMU/SPDK vhost data flow](img/qemu_vhost_data_flow.svg)
The diagram, and the vhost protocol itself is described in @ref vhost_processing doc.
SPDK provides an accelerated vhost target by applying the same user space and polling
techniques as other components in SPDK. Since SPDK is polling for virtio submissions,
it can signal the virtio driver to skip notifications on submission. This avoids VMEXITs on I/O
submission and can significantly reduce CPU usage in the guest VM on heavy I/O workloads.
The following diagram presents how QEMU-based VM communicates with an SPDK vhost device.
+-------QEMU-VM--------+ +---------------SPDK-vhost-------------+
| | | |
| +----------------+ | | +--------------------------------+ |
| | | | | | | |
| | Virtio-SCSI | | eventfd | | +-------------+ | |
| | Linux driver | | interrupt | | Virtio-SCSI | | | |
| | | <----------------+ device | NVMe disk | | |
| +--------^-------+ | | | | | | |
| | | | | +-------^-----+ | |
+----------------------+ | | | | |
| | +----------^---------------------+ |
| | | | |
| +--------------------------------------+
| | |
| polling | | DMA
| | |
+-----------v----Shared hugepage memory------------+------------------------+
| | |
| +----------------------------------+-------------------------v--------+ |
+ | Virtqueues | Buffers | |
| +----------------------------------+----------------------------------+ |
| |
+---------------------------------------------------------------------------+
techniques as other components in SPDK. Since SPDK is polling for vhost submissions,
it can signal the VM to skip notifications on submission. This avoids VMEXITs on I/O
submission and can significantly reduce CPU usage in the VM on heavy I/O workloads.
# Prerequisites {#vhost_prereqs}
@ -135,7 +113,7 @@ will create a 64MB malloc bdev with 512-byte block size.
scripts/rpc.py construct_malloc_bdev 64 512 -b Malloc0
~~~
## Create a virtio device {#vhost_vdev_create}
## Create a vhost device {#vhost_vdev_create}
### Vhost-SCSI