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# Virtio driver {#virtio}
# Introduction {#virtio_intro}
SPDK Virtio driver is a C library that allows communicating with Virtio devices.
It allows any SPDK application to become an initiator for (SPDK) vhost targets.
The driver supports two different usage models:
* PCI - This is the standard mode of operation when used in a guest virtual
machine, where QEMU has presented the virtio controller as a virtual PCI device.
* vhost-user - Can be used to connect to a vhost socket directly on the same host.
The driver, just like the SPDK @ref vhost, is using pollers instead of standard
interrupts to check for an I/O response. If used inside a VM, it bypasses interrupt
and context switching overhead of QEMU and guest kernel, significantly boosting
the overall I/O performance.
This Virtio library is currently used to implement two bdev modules:
@ref bdev_config_virtio_scsi and @ref bdev_config_virtio_blk.
These modules will export generic SPDK block devices usable by any SPDK application.
# 2MB hugepages {#virtio_2mb}
vhost-user specification puts a limitation on the number of "memory regions" used (8).
Each region corresponds to one file descriptor, and DPDK - as SPDK's memory allocator -
uses one file per hugepage by default. So *by default* this makes SPDK Virtio practical
with only 1GB hugepages. To run an SPDK app using Virtio initiator with 2MB hugepages
it is required to pass '-g' command-line option . This forces DPDK to create a single
non-physically-contiguous hugetlbfs file for all its memory.
This functionality requires latest DPDK changes that are officially landing in DPDK
18.05, but have been also backported to spdk-18.02 branch of our internal DPDK fork
which is currently used as a default git submodule for SPDK.
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