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Date: Tue, 19 Mar 2019 14:45:45 +0200
Subject: [PATCH 0/9] RFC: NVME VFIO mediated device

Hi everyone!

In this patch series, I would like to introduce my take on the problem of doing
as fast as possible virtualization of storage with emphasis on low latency.

In this patch series I implemented a kernel vfio based, mediated device that
allows the user to pass through a partition and/or whole namespace to a guest.

The idea behind this driver is based on paper you can find at,

Although note that I stared the development prior to reading this paper,

In addition to that implementation is not based on code used in the paper as
I wasn't being able at that time to make the source available to me.

***Key points about the implementation:***

* Polling kernel thread is used. The polling is stopped after a
predefined timeout (1/2 sec by default).
Support for all interrupt driven mode is planned, and it shows promising results.

* Guest sees a standard NVME device - this allows to run guest with
unmodified drivers, for example windows guests.

* The NVMe device is shared between host and guest.
That means that even a single namespace can be split between host
and guest based on different partitions.

* Simple configuration

*** Performance ***

Performance was tested on Intel DC P3700, With Xeon E5-2620 v2
and both latency and throughput is very similar to SPDK.

Soon I will test this on a better server and nvme device and provide
more formal performance numbers.

Latency numbers:
~80ms - spdk with fio plugin on the host.
~84ms - nvme driver on the host
~87ms - mdev-nvme + nvme driver in the guest

Throughput was following similar pattern as well.

* Configuration example
$ modprobe nvme mdev_queues=4
$ modprobe nvme-mdev

$ UUID=$(uuidgen)
$ DEVICE='device pci address'
$ echo $UUID > /sys/bus/pci/devices/$DEVICE/mdev_supported_types/nvme-2Q_V1/create
$ echo n1p3 > /sys/bus/mdev/devices/$UUID/namespaces/add_namespace #attach host namespace 1 parition 3
$ echo 11 > /sys/bus/mdev/devices/$UUID/settings/iothread_cpu #pin the io thread to cpu 11

Afterward boot qemu with
-device vfio-pci,sysfsdev=/sys/bus/mdev/devices/$UUID

Zero configuration on the guest.

*** FAQ ***

* Why to make this in the kernel? Why this is better that SPDK

-> Reuse the existing nvme kernel driver in the host. No new drivers in the guest.

-> Share the NVMe device between host and guest.
Even in fully virtualized configurations,
some partitions of nvme device could be used by guests as block devices
while others passed through with nvme-mdev to achieve balance between
all features of full IO stack emulation and performance.

-> NVME-MDEV is a bit faster due to the fact that in-kernel driver
can send interrupts to the guest directly without a context
switch that can be expensive due to meltdown mitigation.

-> Is able to utilize interrupts to get reasonable performance.
This is only implemented
as a proof of concept and not included in the patches,
but interrupt driven mode shows reasonable performance

-> This is a framework that later can be used to support NVMe devices
with more of the IO virtualization built-in
(IOMMU with PASID support coupled with device that supports it)

* Why to attach directly to nvme-pci driver and not use block layer IO
-> The direct attachment allows for better performance, but I will
check the possibility of using block IO, especially for fabrics drivers.

*** Implementation notes ***

* All guest memory is mapped into the physical nvme device
but not 1:1 as vfio-pci would do this.
This allows very efficient DMA.
To support this, patch 2 adds ability for a mdev device to listen on
guest's memory map events.
Any such memory is immediately pinned and then DMA mapped.
(Support for fabric drivers where this is not possible exits too,
in which case the fabric driver will do its own DMA mapping)

* nvme core driver is modified to announce the appearance
and disappearance of nvme controllers and namespaces,
to which the nvme-mdev driver is subscribed.

* nvme-pci driver is modified to expose raw interface of attaching to
and sending/polling the IO queues.
This allows the mdev driver very efficiently to submit/poll for the IO.
By default one host queue is used per each mediated device.
(support for other fabric based host drivers is planned)

* The nvme-mdev doesn't assume presence of KVM, thus any VFIO user, including
SPDK, a qemu running with tccg, ... can use this virtual device.

*** Testing ***

The device was tested with stock QEMU 3.0 on the host,
with host was using 5.0 kernel with nvme-mdev added and the following hardware:
* QEMU nvme virtual device (with nested guest)
* Intel DC P3700 on Xeon E5-2620 v2 server
* Samsung SM981 (in a Thunderbolt enclosure, with my laptop)
* Lenovo NVME device found in my laptop

The guest was tested with kernel 4.16, 4.18, 4.20 and
the same custom complied kernel 5.0
Windows 10 guest was tested too with both Microsoft's inbox driver and
open source community NVME driver

Testing was mostly done on x86_64, but 32 bit host/guest combination
was lightly tested too.

In addition to that, the virtual device was tested with nested guest,
by passing the virtual device to it,
using pci passthrough, qemu userspace nvme driver, and spdk

PS: I used to contribute to the kernel as a hobby using the address

Maxim Levitsky (9):
vfio/mdev: add .request callback
nvme/core: add some more values from the spec
nvme/core: add NVME_CTRL_SUSPENDED controller state
nvme/pci: use the NVME_CTRL_SUSPENDED state
nvme/pci: add known admin effects to augument admin effects log page
nvme/pci: init shadow doorbell after each reset
nvme/core: add mdev interfaces
nvme/core: add nvme-mdev core driver
nvme/pci: implement the mdev external queue allocation interface

drivers/nvme/Kconfig | 1 +
drivers/nvme/Makefile | 1 +
drivers/nvme/host/core.c | 149 +++++-
drivers/nvme/host/nvme.h | 55 ++-
drivers/nvme/host/pci.c | 385 ++++++++++++++-
drivers/nvme/mdev/Kconfig | 16 +
drivers/nvme/mdev/Makefile | 5 +
drivers/nvme/mdev/adm.c | 873 ++++++++++++++++++++++++++++++++++
drivers/nvme/mdev/events.c | 142 ++++++
drivers/nvme/mdev/host.c | 491 +++++++++++++++++++
drivers/nvme/mdev/instance.c | 802 +++++++++++++++++++++++++++++++
drivers/nvme/mdev/io.c | 563 ++++++++++++++++++++++
drivers/nvme/mdev/irq.c | 264 ++++++++++
drivers/nvme/mdev/mdev.h | 56 +++
drivers/nvme/mdev/mmio.c | 591 +++++++++++++++++++++++
drivers/nvme/mdev/pci.c | 247 ++++++++++
drivers/nvme/mdev/priv.h | 700 +++++++++++++++++++++++++++
drivers/nvme/mdev/udata.c | 390 +++++++++++++++
drivers/nvme/mdev/vcq.c | 207 ++++++++
drivers/nvme/mdev/vctrl.c | 514 ++++++++++++++++++++
drivers/nvme/mdev/viommu.c | 322 +++++++++++++
drivers/nvme/mdev/vns.c | 356 ++++++++++++++
drivers/nvme/mdev/vsq.c | 178 +++++++
drivers/vfio/mdev/vfio_mdev.c | 11 +
include/linux/mdev.h | 4 +
include/linux/nvme.h | 88 +++-
27 files changed, 7375 insertions(+), 41 deletions(-)
create mode 100644 drivers/nvme/mdev/Kconfig
create mode 100644 drivers/nvme/mdev/Makefile
create mode 100644 drivers/nvme/mdev/adm.c
create mode 100644 drivers/nvme/mdev/events.c
create mode 100644 drivers/nvme/mdev/host.c
create mode 100644 drivers/nvme/mdev/instance.c
create mode 100644 drivers/nvme/mdev/io.c
create mode 100644 drivers/nvme/mdev/irq.c
create mode 100644 drivers/nvme/mdev/mdev.h
create mode 100644 drivers/nvme/mdev/mmio.c
create mode 100644 drivers/nvme/mdev/pci.c
create mode 100644 drivers/nvme/mdev/priv.h
create mode 100644 drivers/nvme/mdev/udata.c
create mode 100644 drivers/nvme/mdev/vcq.c
create mode 100644 drivers/nvme/mdev/vctrl.c
create mode 100644 drivers/nvme/mdev/viommu.c
create mode 100644 drivers/nvme/mdev/vns.c
create mode 100644 drivers/nvme/mdev/vsq.c


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