docs/system/devices/nvme.rst - qemu - Git at Google

 ==============
 NVMe Emulation
 ==============

 QEMU provides NVMe emulation through the ``nvme``, ``nvme-ns`` and
 ``nvme-subsys`` devices.

 See the following sections for specific information on

   * `Adding NVMe Devices`_, `additional namespaces`_ and `NVM subsystems`_.
   * Configuration of `Optional Features`_ such as `Controller Memory Buffer`_,
     `Simple Copy`_, `Zoned Namespaces`_, `metadata`_ and `End-to-End Data
     Protection`_,

 Adding NVMe Devices
 ===================

 Controller Emulation
 --------------------

 The QEMU emulated NVMe controller implements version 1.4 of the NVM Express
 specification. All mandatory features are implement with a couple of exceptions
 and limitations:

   * Accounting numbers in the SMART/Health log page are reset when the device
     is power cycled.
   * Interrupt Coalescing is not supported and is disabled by default.

 The simplest way to attach an NVMe controller on the QEMU PCI bus is to add the
 following parameters:

 .. code-block:: console

     -drive file=nvm.img,if=none,id=nvm
     -device nvme,serial=deadbeef,drive=nvm

 There are a number of optional general parameters for the ``nvme`` device. Some
 are mentioned here, but see ``-device nvme,help`` to list all possible
 parameters.

 ``max_ioqpairs=UINT32`` (default: ``64``)
   Set the maximum number of allowed I/O queue pairs. This replaces the
   deprecated ``num_queues`` parameter.

 ``msix_qsize=UINT16`` (default: ``65``)
   The number of MSI-X vectors that the device should support.

 ``mdts=UINT8`` (default: ``7``)
   Set the Maximum Data Transfer Size of the device.

 ``use-intel-id`` (default: ``off``)
   Since QEMU 5.2, the device uses a QEMU allocated "Red Hat" PCI Device and
   Vendor ID. Set this to ``on`` to revert to the unallocated Intel ID
   previously used.

 Additional Namespaces
 ---------------------

 In the simplest possible invocation sketched above, the device only support a
 single namespace with the namespace identifier ``1``. To support multiple
 namespaces and additional features, the ``nvme-ns`` device must be used.

 .. code-block:: console

    -device nvme,id=nvme-ctrl-0,serial=deadbeef
    -drive file=nvm-1.img,if=none,id=nvm-1
    -device nvme-ns,drive=nvm-1
    -drive file=nvm-2.img,if=none,id=nvm-2
    -device nvme-ns,drive=nvm-2

 The namespaces defined by the ``nvme-ns`` device will attach to the most
 recently defined ``nvme-bus`` that is created by the ``nvme`` device. Namespace
 identifiers are allocated automatically, starting from ``1``.

 There are a number of parameters available:

 ``nsid`` (default: ``0``)
   Explicitly set the namespace identifier.

 ``uuid`` (default: *autogenerated*)
   Set the UUID of the namespace. This will be reported as a "Namespace UUID"
   descriptor in the Namespace Identification Descriptor List.

 ``eui64``
   Set the EUI-64 of the namespace. This will be reported as a "IEEE Extended
   Unique Identifier" descriptor in the Namespace Identification Descriptor List.
   Since machine type 6.1 a non-zero default value is used if the parameter
   is not provided. For earlier machine types the field defaults to 0.

 ``bus``
   If there are more ``nvme`` devices defined, this parameter may be used to
   attach the namespace to a specific ``nvme`` device (identified by an ``id``
   parameter on the controller device).

 NVM Subsystems
 --------------

 Additional features becomes available if the controller device (``nvme``) is
 linked to an NVM Subsystem device (``nvme-subsys``).

 The NVM Subsystem emulation allows features such as shared namespaces and
 multipath I/O.

 .. code-block:: console

    -device nvme-subsys,id=nvme-subsys-0,nqn=subsys0
    -device nvme,serial=deadbeef,subsys=nvme-subsys-0
    -device nvme,serial=deadbeef,subsys=nvme-subsys-0

 This will create an NVM subsystem with two controllers. Having controllers
 linked to an ``nvme-subsys`` device allows additional ``nvme-ns`` parameters:

 ``shared`` (default: ``on`` since 6.2)
   Specifies that the namespace will be attached to all controllers in the
   subsystem. If set to ``off``, the namespace will remain a private namespace
   and may only be attached to a single controller at a time. Shared namespaces
   are always automatically attached to all controllers (also when controllers
   are hotplugged).

 ``detached`` (default: ``off``)
   If set to ``on``, the namespace will be be available in the subsystem, but
   not attached to any controllers initially. A shared namespace with this set
   to ``on`` will never be automatically attached to controllers.

 Thus, adding

 .. code-block:: console

    -drive file=nvm-1.img,if=none,id=nvm-1
    -device nvme-ns,drive=nvm-1,nsid=1
    -drive file=nvm-2.img,if=none,id=nvm-2
    -device nvme-ns,drive=nvm-2,nsid=3,shared=off,detached=on

 will cause NSID 1 will be a shared namespace that is initially attached to both
 controllers. NSID 3 will be a private namespace due to ``shared=off`` and only
 attachable to a single controller at a time. Additionally it will not be
 attached to any controller initially (due to ``detached=on``) or to hotplugged
 controllers.

 Optional Features
 =================

 Controller Memory Buffer
 ------------------------

 ``nvme`` device parameters related to the Controller Memory Buffer support:

 ``cmb_size_mb=UINT32`` (default: ``0``)
   This adds a Controller Memory Buffer of the given size at offset zero in BAR
   2.

 ``legacy-cmb`` (default: ``off``)
   By default, the device uses the "v1.4 scheme" for the Controller Memory
   Buffer support (i.e, the CMB is initially disabled and must be explicitly
   enabled by the host). Set this to ``on`` to behave as a v1.3 device wrt. the
   CMB.

 Simple Copy
 -----------

 The device includes support for TP 4065 ("Simple Copy Command"). A number of
 additional ``nvme-ns`` device parameters may be used to control the Copy
 command limits:

 ``mssrl=UINT16`` (default: ``128``)
   Set the Maximum Single Source Range Length (``MSSRL``). This is the maximum
   number of logical blocks that may be specified in each source range.

 ``mcl=UINT32`` (default: ``128``)
   Set the Maximum Copy Length (``MCL``). This is the maximum number of logical
   blocks that may be specified in a Copy command (the total for all source
   ranges).

 ``msrc=UINT8`` (default: ``127``)
   Set the Maximum Source Range Count (``MSRC``). This is the maximum number of
   source ranges that may be used in a Copy command. This is a 0's based value.

 Zoned Namespaces
 ----------------

 A namespaces may be "Zoned" as defined by TP 4053 ("Zoned Namespaces"). Set
 ``zoned=on`` on an ``nvme-ns`` device to configure it as a zoned namespace.

 The namespace may be configured with additional parameters

 ``zoned.zone_size=SIZE`` (default: ``128MiB``)
   Define the zone size (``ZSZE``).

 ``zoned.zone_capacity=SIZE`` (default: ``0``)
   Define the zone capacity (``ZCAP``). If left at the default (``0``), the zone
   capacity will equal the zone size.

 ``zoned.descr_ext_size=UINT32`` (default: ``0``)
   Set the Zone Descriptor Extension Size (``ZDES``). Must be a multiple of 64
   bytes.

 ``zoned.cross_read=BOOL`` (default: ``off``)
   Set to ``on`` to allow reads to cross zone boundaries.

 ``zoned.max_active=UINT32`` (default: ``0``)
   Set the maximum number of active resources (``MAR``). The default (``0``)
   allows all zones to be active.

 ``zoned.max_open=UINT32`` (default: ``0``)
   Set the maximum number of open resources (``MOR``). The default (``0``)
   allows all zones to be open. If ``zoned.max_active`` is specified, this value
   must be less than or equal to that.

 ``zoned.zasl=UINT8`` (default: ``0``)
   Set the maximum data transfer size for the Zone Append command. Like
   ``mdts``, the value is specified as a power of two (2^n) and is in units of
   the minimum memory page size (CAP.MPSMIN). The default value (``0``)
   has this property inherit the ``mdts`` value.

 Flexible Data Placement
 -----------------------

 The device may be configured to support TP4146 ("Flexible Data Placement") by
 configuring it (``fdp=on``) on the subsystem::

     -device nvme-subsys,id=nvme-subsys-0,nqn=subsys0,fdp=on,fdp.nruh=16

 The subsystem emulates a single Endurance Group, on which Flexible Data
 Placement will be supported. Also note that the device emulation deviates
 slightly from the specification, by always enabling the "FDP Mode" feature on
 the controller if the subsystems is configured for Flexible Data Placement.

 Enabling Flexible Data Placement on the subsyste enables the following
 parameters:

 ``fdp.nrg`` (default: ``1``)
   Set the number of Reclaim Groups.

 ``fdp.nruh`` (default: ``0``)
   Set the number of Reclaim Unit Handles. This is a mandatory paramater and
   must be non-zero.

 ``fdp.runs`` (default: ``96M``)
   Set the Reclaim Unit Nominal Size. Defaults to 96 MiB.

 Namespaces within this subsystem may requests Reclaim Unit Handles::

     -device nvme-ns,drive=nvm-1,fdp.ruhs=RUHLIST

 The ``RUHLIST`` is a semicolon separated list (i.e. ``0;1;2;3``) and may
 include ranges (i.e. ``0;8-15``). If no reclaim unit handle list is specified,
 the controller will assign the controller-specified reclaim unit handle to
 placement handle identifier 0.

 Metadata
 --------

 The virtual namespace device supports LBA metadata in the form separate
 metadata (``MPTR``-based) and extended LBAs.

 ``ms=UINT16`` (default: ``0``)
   Defines the number of metadata bytes per LBA.

 ``mset=UINT8`` (default: ``0``)
   Set to ``1`` to enable extended LBAs.

 End-to-End Data Protection
 --------------------------

 The virtual namespace device supports DIF- and DIX-based protection information
 (depending on ``mset``).

 ``pi=UINT8`` (default: ``0``)
   Enable protection information of the specified type (type ``1``, ``2`` or
   ``3``).

 ``pil=UINT8`` (default: ``0``)
   Controls the location of the protection information within the metadata. Set
   to ``1`` to transfer protection information as the first eight bytes of
   metadata. Otherwise, the protection information is transferred as the last
   eight bytes.

 Virtualization Enhancements and SR-IOV (Experimental Support)
 -------------------------------------------------------------

 The ``nvme`` device supports Single Root I/O Virtualization and Sharing
 along with Virtualization Enhancements. The controller has to be linked to
 an NVM Subsystem device (``nvme-subsys``) for use with SR-IOV.

 A number of parameters are present (**please note, that they may be
 subject to change**):

 ``sriov_max_vfs`` (default: ``0``)
   Indicates the maximum number of PCIe virtual functions supported
   by the controller. Specifying a non-zero value enables reporting of both
   SR-IOV and ARI (Alternative Routing-ID Interpretation) capabilities
   by the NVMe device. Virtual function controllers will not report SR-IOV.

 ``sriov_vq_flexible``
   Indicates the total number of flexible queue resources assignable to all
   the secondary controllers. Implicitly sets the number of primary
   controller's private resources to ``(max_ioqpairs - sriov_vq_flexible)``.

 ``sriov_vi_flexible``
   Indicates the total number of flexible interrupt resources assignable to
   all the secondary controllers. Implicitly sets the number of primary
   controller's private resources to ``(msix_qsize - sriov_vi_flexible)``.

 ``sriov_max_vi_per_vf`` (default: ``0``)
   Indicates the maximum number of virtual interrupt resources assignable
   to a secondary controller. The default ``0`` resolves to
   ``(sriov_vi_flexible / sriov_max_vfs)``

 ``sriov_max_vq_per_vf`` (default: ``0``)
   Indicates the maximum number of virtual queue resources assignable to
   a secondary controller. The default ``0`` resolves to
   ``(sriov_vq_flexible / sriov_max_vfs)``

 The simplest possible invocation enables the capability to set up one VF
 controller and assign an admin queue, an IO queue, and a MSI-X interrupt.

 .. code-block:: console

    -device nvme-subsys,id=subsys0
    -device nvme,serial=deadbeef,subsys=subsys0,sriov_max_vfs=1,
     sriov_vq_flexible=2,sriov_vi_flexible=1

 The minimum steps required to configure a functional NVMe secondary
 controller are:

   * unbind flexible resources from the primary controller

 .. code-block:: console

    nvme virt-mgmt /dev/nvme0 -c 0 -r 1 -a 1 -n 0
    nvme virt-mgmt /dev/nvme0 -c 0 -r 0 -a 1 -n 0

   * perform a Function Level Reset on the primary controller to actually
     release the resources

 .. code-block:: console

    echo 1 > /sys/bus/pci/devices/0000:01:00.0/reset

   * enable VF

 .. code-block:: console

    echo 1 > /sys/bus/pci/devices/0000:01:00.0/sriov_numvfs

   * assign the flexible resources to the VF and set it ONLINE

 .. code-block:: console

    nvme virt-mgmt /dev/nvme0 -c 1 -r 1 -a 8 -n 1
    nvme virt-mgmt /dev/nvme0 -c 1 -r 0 -a 8 -n 2
    nvme virt-mgmt /dev/nvme0 -c 1 -r 0 -a 9 -n 0

   * bind the NVMe driver to the VF

 .. code-block:: console

    echo 0000:01:00.1 > /sys/bus/pci/drivers/nvme/bind
	==============
	NVMe Emulation
	==============

	QEMU provides NVMe emulation through the ``nvme``, ``nvme-ns`` and
	``nvme-subsys`` devices.

	See the following sections for specific information on

	* `Adding NVMe Devices`_, `additional namespaces`_ and `NVM subsystems`_.
	* Configuration of `Optional Features`_ such as `Controller Memory Buffer`_,
	`Simple Copy`_, `Zoned Namespaces`_, `metadata`_ and `End-to-End Data
	Protection`_,

	Adding NVMe Devices
	===================

	Controller Emulation
	--------------------

	The QEMU emulated NVMe controller implements version 1.4 of the NVM Express
	specification. All mandatory features are implement with a couple of exceptions
	and limitations:

	* Accounting numbers in the SMART/Health log page are reset when the device
	is power cycled.
	* Interrupt Coalescing is not supported and is disabled by default.

	The simplest way to attach an NVMe controller on the QEMU PCI bus is to add the
	following parameters:

	.. code-block:: console

	-drive file=nvm.img,if=none,id=nvm
	-device nvme,serial=deadbeef,drive=nvm

	There are a number of optional general parameters for the ``nvme`` device. Some
	are mentioned here, but see ``-device nvme,help`` to list all possible
	parameters.

	``max_ioqpairs=UINT32`` (default: ``64``)
	Set the maximum number of allowed I/O queue pairs. This replaces the
	deprecated ``num_queues`` parameter.

	``msix_qsize=UINT16`` (default: ``65``)
	The number of MSI-X vectors that the device should support.

	``mdts=UINT8`` (default: ``7``)
	Set the Maximum Data Transfer Size of the device.

	``use-intel-id`` (default: ``off``)
	Since QEMU 5.2, the device uses a QEMU allocated "Red Hat" PCI Device and
	Vendor ID. Set this to ``on`` to revert to the unallocated Intel ID
	previously used.

	Additional Namespaces
	---------------------

	In the simplest possible invocation sketched above, the device only support a
	single namespace with the namespace identifier ``1``. To support multiple
	namespaces and additional features, the ``nvme-ns`` device must be used.

	.. code-block:: console

	-device nvme,id=nvme-ctrl-0,serial=deadbeef
	-drive file=nvm-1.img,if=none,id=nvm-1
	-device nvme-ns,drive=nvm-1
	-drive file=nvm-2.img,if=none,id=nvm-2
	-device nvme-ns,drive=nvm-2

	The namespaces defined by the ``nvme-ns`` device will attach to the most
	recently defined ``nvme-bus`` that is created by the ``nvme`` device. Namespace
	identifiers are allocated automatically, starting from ``1``.

	There are a number of parameters available:

	``nsid`` (default: ``0``)
	Explicitly set the namespace identifier.

	``uuid`` (default: autogenerated)
	Set the UUID of the namespace. This will be reported as a "Namespace UUID"
	descriptor in the Namespace Identification Descriptor List.

	``eui64``
	Set the EUI-64 of the namespace. This will be reported as a "IEEE Extended
	Unique Identifier" descriptor in the Namespace Identification Descriptor List.
	Since machine type 6.1 a non-zero default value is used if the parameter
	is not provided. For earlier machine types the field defaults to 0.

	``bus``
	If there are more ``nvme`` devices defined, this parameter may be used to
	attach the namespace to a specific ``nvme`` device (identified by an ``id``
	parameter on the controller device).

	NVM Subsystems
	--------------

	Additional features becomes available if the controller device (``nvme``) is
	linked to an NVM Subsystem device (``nvme-subsys``).

	The NVM Subsystem emulation allows features such as shared namespaces and
	multipath I/O.

	.. code-block:: console

	-device nvme-subsys,id=nvme-subsys-0,nqn=subsys0
	-device nvme,serial=deadbeef,subsys=nvme-subsys-0
	-device nvme,serial=deadbeef,subsys=nvme-subsys-0

	This will create an NVM subsystem with two controllers. Having controllers
	linked to an ``nvme-subsys`` device allows additional ``nvme-ns`` parameters:

	``shared`` (default: ``on`` since 6.2)
	Specifies that the namespace will be attached to all controllers in the
	subsystem. If set to ``off``, the namespace will remain a private namespace
	and may only be attached to a single controller at a time. Shared namespaces
	are always automatically attached to all controllers (also when controllers
	are hotplugged).

	``detached`` (default: ``off``)
	If set to ``on``, the namespace will be be available in the subsystem, but
	not attached to any controllers initially. A shared namespace with this set
	to ``on`` will never be automatically attached to controllers.

	Thus, adding

	.. code-block:: console

	-drive file=nvm-1.img,if=none,id=nvm-1
	-device nvme-ns,drive=nvm-1,nsid=1
	-drive file=nvm-2.img,if=none,id=nvm-2
	-device nvme-ns,drive=nvm-2,nsid=3,shared=off,detached=on

	will cause NSID 1 will be a shared namespace that is initially attached to both
	controllers. NSID 3 will be a private namespace due to ``shared=off`` and only
	attachable to a single controller at a time. Additionally it will not be
	attached to any controller initially (due to ``detached=on``) or to hotplugged
	controllers.

	Optional Features
	=================

	Controller Memory Buffer
	------------------------

	``nvme`` device parameters related to the Controller Memory Buffer support:

	``cmb_size_mb=UINT32`` (default: ``0``)
	This adds a Controller Memory Buffer of the given size at offset zero in BAR
	2.

	``legacy-cmb`` (default: ``off``)
	By default, the device uses the "v1.4 scheme" for the Controller Memory
	Buffer support (i.e, the CMB is initially disabled and must be explicitly
	enabled by the host). Set this to ``on`` to behave as a v1.3 device wrt. the
	CMB.

	Simple Copy
	-----------

	The device includes support for TP 4065 ("Simple Copy Command"). A number of
	additional ``nvme-ns`` device parameters may be used to control the Copy
	command limits:

	``mssrl=UINT16`` (default: ``128``)
	Set the Maximum Single Source Range Length (``MSSRL``). This is the maximum
	number of logical blocks that may be specified in each source range.

	``mcl=UINT32`` (default: ``128``)
	Set the Maximum Copy Length (``MCL``). This is the maximum number of logical
	blocks that may be specified in a Copy command (the total for all source
	ranges).

	``msrc=UINT8`` (default: ``127``)
	Set the Maximum Source Range Count (``MSRC``). This is the maximum number of
	source ranges that may be used in a Copy command. This is a 0's based value.

	Zoned Namespaces
	----------------

	A namespaces may be "Zoned" as defined by TP 4053 ("Zoned Namespaces"). Set
	``zoned=on`` on an ``nvme-ns`` device to configure it as a zoned namespace.

	The namespace may be configured with additional parameters

	``zoned.zone_size=SIZE`` (default: ``128MiB``)
	Define the zone size (``ZSZE``).

	``zoned.zone_capacity=SIZE`` (default: ``0``)
	Define the zone capacity (``ZCAP``). If left at the default (``0``), the zone
	capacity will equal the zone size.

	``zoned.descr_ext_size=UINT32`` (default: ``0``)
	Set the Zone Descriptor Extension Size (``ZDES``). Must be a multiple of 64
	bytes.

	``zoned.cross_read=BOOL`` (default: ``off``)
	Set to ``on`` to allow reads to cross zone boundaries.

	``zoned.max_active=UINT32`` (default: ``0``)
	Set the maximum number of active resources (``MAR``). The default (``0``)
	allows all zones to be active.

	``zoned.max_open=UINT32`` (default: ``0``)
	Set the maximum number of open resources (``MOR``). The default (``0``)
	allows all zones to be open. If ``zoned.max_active`` is specified, this value
	must be less than or equal to that.

	``zoned.zasl=UINT8`` (default: ``0``)
	Set the maximum data transfer size for the Zone Append command. Like
	``mdts``, the value is specified as a power of two (2^n) and is in units of
	the minimum memory page size (CAP.MPSMIN). The default value (``0``)
	has this property inherit the ``mdts`` value.

	Flexible Data Placement
	-----------------------

	The device may be configured to support TP4146 ("Flexible Data Placement") by
	configuring it (``fdp=on``) on the subsystem::

	-device nvme-subsys,id=nvme-subsys-0,nqn=subsys0,fdp=on,fdp.nruh=16

	The subsystem emulates a single Endurance Group, on which Flexible Data
	Placement will be supported. Also note that the device emulation deviates
	slightly from the specification, by always enabling the "FDP Mode" feature on
	the controller if the subsystems is configured for Flexible Data Placement.

	Enabling Flexible Data Placement on the subsyste enables the following
	parameters:

	``fdp.nrg`` (default: ``1``)
	Set the number of Reclaim Groups.

	``fdp.nruh`` (default: ``0``)
	Set the number of Reclaim Unit Handles. This is a mandatory paramater and
	must be non-zero.

	``fdp.runs`` (default: ``96M``)
	Set the Reclaim Unit Nominal Size. Defaults to 96 MiB.

	Namespaces within this subsystem may requests Reclaim Unit Handles::

	-device nvme-ns,drive=nvm-1,fdp.ruhs=RUHLIST

	The ``RUHLIST`` is a semicolon separated list (i.e. ``0;1;2;3``) and may
	include ranges (i.e. ``0;8-15``). If no reclaim unit handle list is specified,
	the controller will assign the controller-specified reclaim unit handle to
	placement handle identifier 0.

	Metadata
	--------

	The virtual namespace device supports LBA metadata in the form separate
	metadata (``MPTR``-based) and extended LBAs.

	``ms=UINT16`` (default: ``0``)
	Defines the number of metadata bytes per LBA.

	``mset=UINT8`` (default: ``0``)
	Set to ``1`` to enable extended LBAs.

	End-to-End Data Protection
	--------------------------

	The virtual namespace device supports DIF- and DIX-based protection information
	(depending on ``mset``).

	``pi=UINT8`` (default: ``0``)
	Enable protection information of the specified type (type ``1``, ``2`` or
	``3``).

	``pil=UINT8`` (default: ``0``)
	Controls the location of the protection information within the metadata. Set
	to ``1`` to transfer protection information as the first eight bytes of
	metadata. Otherwise, the protection information is transferred as the last
	eight bytes.

	Virtualization Enhancements and SR-IOV (Experimental Support)
	-------------------------------------------------------------

	The ``nvme`` device supports Single Root I/O Virtualization and Sharing
	along with Virtualization Enhancements. The controller has to be linked to
	an NVM Subsystem device (``nvme-subsys``) for use with SR-IOV.

	A number of parameters are present (**please note, that they may be
	subject to change**):

	``sriov_max_vfs`` (default: ``0``)
	Indicates the maximum number of PCIe virtual functions supported
	by the controller. Specifying a non-zero value enables reporting of both
	SR-IOV and ARI (Alternative Routing-ID Interpretation) capabilities
	by the NVMe device. Virtual function controllers will not report SR-IOV.

	``sriov_vq_flexible``
	Indicates the total number of flexible queue resources assignable to all
	the secondary controllers. Implicitly sets the number of primary
	controller's private resources to ``(max_ioqpairs - sriov_vq_flexible)``.

	``sriov_vi_flexible``
	Indicates the total number of flexible interrupt resources assignable to
	all the secondary controllers. Implicitly sets the number of primary
	controller's private resources to ``(msix_qsize - sriov_vi_flexible)``.

	``sriov_max_vi_per_vf`` (default: ``0``)
	Indicates the maximum number of virtual interrupt resources assignable
	to a secondary controller. The default ``0`` resolves to
	``(sriov_vi_flexible / sriov_max_vfs)``

	``sriov_max_vq_per_vf`` (default: ``0``)
	Indicates the maximum number of virtual queue resources assignable to
	a secondary controller. The default ``0`` resolves to
	``(sriov_vq_flexible / sriov_max_vfs)``

	The simplest possible invocation enables the capability to set up one VF
	controller and assign an admin queue, an IO queue, and a MSI-X interrupt.

	.. code-block:: console

	-device nvme-subsys,id=subsys0
	-device nvme,serial=deadbeef,subsys=subsys0,sriov_max_vfs=1,
	sriov_vq_flexible=2,sriov_vi_flexible=1

	The minimum steps required to configure a functional NVMe secondary
	controller are:

	* unbind flexible resources from the primary controller

	.. code-block:: console

	nvme virt-mgmt /dev/nvme0 -c 0 -r 1 -a 1 -n 0
	nvme virt-mgmt /dev/nvme0 -c 0 -r 0 -a 1 -n 0

	* perform a Function Level Reset on the primary controller to actually
	release the resources

	.. code-block:: console

	echo 1 > /sys/bus/pci/devices/0000:01:00.0/reset

	* enable VF

	.. code-block:: console

	echo 1 > /sys/bus/pci/devices/0000:01:00.0/sriov_numvfs

	* assign the flexible resources to the VF and set it ONLINE

	.. code-block:: console

	nvme virt-mgmt /dev/nvme0 -c 1 -r 1 -a 8 -n 1
	nvme virt-mgmt /dev/nvme0 -c 1 -r 0 -a 8 -n 2
	nvme virt-mgmt /dev/nvme0 -c 1 -r 0 -a 9 -n 0

	* bind the NVMe driver to the VF

	.. code-block:: console

	echo 0000:01:00.1 > /sys/bus/pci/drivers/nvme/bind