DOCA DevEmu Virtio-FS

The DOCA DevEmu Virtio-FS library is part of the DOCA DevEmu Virtio subsystem. It provides low-level software APIs that provide building blocks for developing and manipulating virtio filesystem devices using the device emulation capability of NVIDIA® BlueField® DPUs.

DOCA supports emulating virtio-FS devices over the PCIe bus. The PCIe transport is the common transport used for virtio devices. Configuration, discovery, and features related to PCIe (e.g., MSI-X and PCIe device hot plug/unplug) are managed through the DOCA DevEmu PCI APIs. Configuring common virtio registers and handling generic virtio logic (e.g., virtio device reset flow) is handled by the DOCA Virtio common library. This modular design enables each layer within the DOCA Device Emulation subsystem to manage its own business logic. It facilitates seamless integration with the other layers, ensuring independent functionality and operation throughout the system.

The DOCA Devemu Virtio-FS library efficiently handles virtio descriptors, carrying FUSE requests, sent by the device driver, and translating them into abstract virtio-FS requests which are then routed to the user. This translation process ensures that the underlying device-specific acceleration details are abstracted away, allowing applications to interact with abstracted virtio-FS requests.

Users of this library are responsible for developing a virtio-FS controller, which manages the underlying DOCA Devemu Virtio-FS device alongside an external backend file system which is outside DOCA's scope. The controller application is designed to receive DOCA Virtio-FS requests and process them according to virtio-FS and FUSE specifications, translating FUSE-based commands into the appropriate backend filesystem protocol.

Virtio-FS device emulation is part of DOCA DevEmu Virtio subsystem. It is, therefore, recommended to read the following guides before proceeding:

• DOCA Device Emulation

• DOCA DevEmu PCI

• DOCA DevEmu Virtio

DOCA DevEmu Virtio-FS is supported on the BlueField target only. The BlueField must meet the following requirements:

• DOCA version 2.7.0 or greater

• BlueField-3 firmware 32.41.1000 or higher

Perform the following:

1. Configure BlueField to work in DPU mode as described in NVIDIA BlueField Modes of Operation.

2. Enable emulation by running the following on the host or DPU:

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   host/dpu> sudo mlxconfig -d /dev/mst/mt41692_pciconf0 s VIRTIO_FS_EMULATION_ENABLE=1
           

3. Configure the number of static virtio-FS physical functions and the number of MSIX for each physical function to expose. This can be done by running the following command on the DPU:

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   host/dpu> sudo mlxconfig -d /dev/mst/mt41692_pciconf0 s VIRTIO_FS_EMULATION_NUM_PF=2 VIRTIO_FS_EMULATION_NUM_MSIX=18
           

4. Perform a BlueField system reboot for the mlxconfig settings to take effect.

The DOCA DevEmu Virtio-FS library provides the following main software abstractions:

• The virtio-FS type – extends the virtio type; represents common/default virtio-FS configurations of emulated virtio-FS devices

• The virtio-FS device – extends the virtio device; represents an instance of an emulated virtio-FS device

• The virtio-FS IO context – extends the virtio IO context; represents a progress context responsible for processing virtio descriptors, carrying FUSE requests, and their associated virtio queues (e.g., hiprio, request, admin, and notification queues).

• The virtio-FS request

Virtio-FS Feature Bits

According to the virtio specification, a virtio-FS device may report support for VIRTIO_FS_F_NOTIFICATION which indicates the ability to handle FUSE notify messages sent via the notification queue.

Virtio-FS Configuration Layout

According to the virtio specification, the virtio-FS configuration structure layout is as follows:

            

            
struct virtio_fs_config {
	char tag[36];
	le32 num_request_queues;
	le32 notify_buf_size;
};
        

The tag and num_request_queues fields are always available. The notify_buf_size field is only available when VIRTIO_FS_F_NOTIFICATION is set.

Virtio-FS Type

The virtio-FS type extends the virtio type and describes the common/default configuration of emulated virtio-FS devices , including some of the virtio-FS configuration space registers (e.g., num_request_queues ).

Currently , the virtio-FS type is read-only (i.e., only getter APIs are available to r etrieve information) . T he following method can be used for this purpose:

• doca_devemu_vfs_type_get_num_request_queues – to get the default initial value of the num_request_queues register for the associated virtio-FS devices

DOCA supports the default virtio-FS type. To retrieve the default virtio-FS type, users use the following method:

• doca_devemu_vfs_is_default_vfs_type_supported – check if the default DOCA Virtio-FS type is supported by the device. If supported:

  • doca_dev_open – open supported DOCA device

  • doca_devemu_vfs_find_default_vfs_type_by_dev – get the default DOCA Virtio-FS type associated with the device

Virtio-FS Device

The virtio-FS device extends the virtio device. Before using the DOCA DevEmu Virtio-FS device, i t is recommended to read the guidelines of DOCA DevEmu Virtio device, DOCA DevEmu PCI device, and DOCA Core context configuration phase .

This section describes how to create, configure, and operate the virtio-FS device.

Virtio-FS Device Configurations

The virtio-FS emulated device might be in several different visibility levels from the host point of view:

• Visible/non-visible to the PCIe subsystem – If the device is visible to the PCIe subsystem, the user is not able to configure PCIe-related parameters (e.g., number of MSI-X vector, subsystem_id).

• Visible/non-visible to the virtio subsystem – If the device is visible to the virtio subsystem, the user is not be able to configure virtio-related parameters (e.g., number of queues, queue_size).

The flow for creating and configuring a virtio-FS device is as follows:

1. doca_devemu_vfs_dev_create – Create a new DOCA DevEmu Virtio-FS device instance.

2. doca_devemu_vfs_dev_set_tag – Set a unique tag for the device according to the virtio specification.

3. doca_devemu_vfs_dev_set_num_request_queues – Set the number of request queues for the device.

4. doca_devemu_vfs_dev_set_vfs_req_user_data_size – Set the user data size of the virtio-FS request. If set, a buffer with this size is allocated for each DOCA DevEmu Virtio-FS on behalf of the user.

5. Configure virtio-related parameters as described in DOCA Virtio configurations.

   Note

   doca_devemu_virtio_dev_set_num_queues should be equal to the number of request queues +1 (for the hiprio queue) since DOCA does not currently support the virtio-FS notification queue.

6. Configure PCIe-related parameters as described in DOCA DevEmu PCI configurations.

7. doca_ctx_start – Start the virtio-FS device context to finalize the configuration phase.

   • The virtio-FS device object follows the DOCA context state machine as described in DOCA Core context state machine

   • The virtio-FS device context moves to running state after the initial number of virtio IO contexts is bound to it and turns to running state, as described at DOCA DevEmu Virtio configurations

At this point, the DOCA Devemu Virtio-FS context is fully operational.

Mandatory Configurations

The following are mandatory configurations:

• doca_devemu_vfs_dev_set_tag – s et a unique tag for the device

Optional Configurations

The optional configurations are as follows:

• doca_devemu_vfs_dev_set_num_request_queues – set the number of request queues for the device. If not set, the default value is taken from the virtio-FS type configuration.

• doca_devemu_vfs_dev_set_vfs_req_user_data_size – set the user data size of the virtio-FS request. If not set, user data size defaults to 0.

Virtio-FS Device Events

DOCA DevEmu Virtio-FS device exposes asynchronous events to notify about changes that happen out of the blue, according to the DOCA Core architecture.

Common events are described in DOCA DevEmu Virtio device events, DOCA DevEmu PCI device events and in DOCA Core event .

Virtio-FS IO

The virtio-FS IO context extends the Virtio IO Context. To start using the DOCA DevEmu Virtio-FS IO i t is recommended to read the guidelines of DOCA DevEmu Virtio IO and DOCA Core context configuration phase.

This section describes how to create, configure and operate the virtio-FS IO context.

Virtio-FS IO Configurations

The flow for creating and configuring a virtio-FS IO context should be as follows:

1. doca_devemu_vfs_io_create – C reate a new DOCA DevEmu Virtio-FS IO instance.

2. doca_devemu_vfs_io_event_vfs_req_notice_register – Register event handler for incoming virtio-FS requests.

3. doca_ctx_start – Start the virtio-FS IO context to finalize the configuration phase. The virtio-FS IO object follows the DOCA Core context state machine. The virtio-FS device context moves to running state after the initial number of virtio-FS IO contexts is bound to it and moves to running state (as described at DOCA DevEmu Virtio configurations).

Mandatory Configurations

The following are mandatory configurations:

• doca_devemu_vfs_io_event_vfs_req_notice_register – Register event handler for incoming virtio-FS requests is mandatory

Virtio-FS Request

The virtio-FS request object serves as an abstraction for handling requests arriving on virtio-FS queues, including high-priority, request, or notification queues. These requests are initially generated by the device driver through created virtio queues and then routed to the user via a registered event handler, which is set up using doca_devemu_vfs_io_event_vfs_req_notice_register , on the associated virtio IO context. This event handler, issued by the DOCA Virtio FS library, ensures that users can receive and process virtio-FS requests effectively within their application. Once the event handler is called, the ownership of the virtio-FS request and the associated request user data move to the user. The request ownership moves back to the associated virtio IO context once it is completed by the user by calling doca_devemu_vfs_req_complete.

The following APIs operate a virtio-FS request:

1. doca_devemu_vfs_req_get_datain – Get a DOCA buffer representing the data-in of the virtio-FS request. This DOCA buffer represents the host memory for the device-readable part of the request according to the virtio specification.

2. doca_devemu_vfs_req_get_dataout – Get a DOCA buffer representing the data-out of the virtio-FS request. This DOCA buffer represents the host memory for the device-writable part of the request according to the virtio specification.

3. doca_devemu_vfs_req_complete – Complete the virtio-FS request. The associated virtio-FS IO context completes the request toward the device driver according to the virtio-FS specification.

Emulated virtio-FS PCIe functions are represented by a doca_devinfo_rep . To find the suitable doca_devinfo_rep that is used as the input parameter for doca_devemu_vfs_dev_create, users should first discover the existing device representors using the below:

1. doca_devinfo_create_list – Get a list of all DOCA devices.

2. doca_devemu_vfs_is_default_vfs_type_supported – Check whether the device can manage device associated to virtio-FS type.

3. If supported:

   1. doca_dev_open – Get an instance of the DOCA device that can be used as virtio-FS emulation manager.

   2. doca_devemu_vfs_find_default_vfs_type_by_dev – Get the default virtio-FS device type.

   3. doca_devemu_vfs_type_as_pci_type – Cast virtio-FS type to PCIe type.

   4. doca_devemu_pci_type_rep_list_create – Create a list of all available representor devices for the virtio-FS type.

4. At this point, the user can choose the preferred representor device, open it using doca_dev_rep_open, and proceed with the flow described in section "Virtio-FS Device Configurations".

This section describes the initialization flow of a DOCA DevEmu Virtio-FS device and one or more DOCA DevEmu Virtio-FS IO contexts (4 in this example). In this procedure, the user sets up and prepares the environment before starting to receive control path events (from the virtio-FS device context) and IO requests (from the virtio-FS IO contexts). During initialization, the user should configure various essential components to ensure correct behavior.

The user should perform the following:

1. Choose 4 Arm cores to run the application threads on.

2. Create 4 DOCA Core progress engine (PE) objects (pe1, pe2 , pe3 , pe4 ).

3. Find the suitable representor device according to the Discovery flow or any other method.

4. Create, configure, and start a new virtio-FS device according to the virtio-FS device configuration flow. Assume pe1 is associated with the virtio-FS device and doca_devemu_virtio_dev_set_num_required_running_virtio_io_ctxs is set to 4.

5. Create, configure, and start 4 new virtio-FS IO contexts according to the virtio-FS IO configuration flow . A ssume pe1, pe2 , pe3 , and pe4 are associated with each of the 4 virtio-FS IO contexts respectively.

6. At this point, the 4 v irtio-FS IO contexts transition to running state, followed by the virtio-FS device context transitioning to running state.

This section describes the teardown flow of DOCA DevEmu Virtio-FS device and one or more DOCA DevEmu Virtio-FS IO contexts (4 in this example). In this procedure, the user cleans all the resources allocated in the initialization flow and all the outstanding events and requests.

The user should perform the following:

1. Start the teardown flow by calling doca_ctx_stop. This causes the DOCA Virtio-FS device context to transition to stopping state. It is guaranteed that no virtio/PCIe control path events is generated during this state.

2. Call doca_ctx_stop for any DOCA Virtio-FS IO context. This causes the DOCA Virtio-FS IO context to transition to stopping state . It is guaranteed that no IO path events are generated during this state.

3. Flush all outstanding virtio-FS requests to the associated virtio-FS IO contexts by calling doca_devemu_vfs_req_complete. Upon completing all the requests associated with a virtio-FS IO context, the DOCA Virtio-FS IO context transitions to idle state.

4. At this point, it is safe to destroy the virtio-FS IO context by calling doca_devemu_vfs_io_destroy. Destroying a virtio-FS IO context not in idle state will fail .

5. Once all 4 virtio-FS IO contexts associated with the virtio-FS device transition to idle state , the DOCA Virtio-FS device context transitions to idle state as well .

6. At this point, it is safe to destroy the virtio-FS device context by calling doca_devemu_vfs_dev_destroy. Destroying a virtio-FS device context not in idle state will fail.

This section describes execution on BlueField Arm cores using several DOCA Core PE objects (one per core):

1. Choose 4 Arm cores to run the application threads on.

2. Create 4 DOCA Core PE objects. The application threads should periodically call doca_pe_progress to advance all DOCA contexts associated with the PE.

3. Create, configure, and start the DOCA Virtio-FS device.

4. Create, configure, and start 4 DOCA Virtio-FS IO contexts.

The progress of DOCA Virtio-FS objects is illustrated by the following diagram:

Control Path

The DOCA Virtio-FS device context extends the DOCA Virtio device context (which extends the DOCA PCIe device context). This means that the DOCA Virtio-FS device control path is comprised by all the object it extends (i.e., DOCA Context, DOCA DevEmu PCI device, and DOCA DevEmu Virtio device).

The following events can be triggered by a virtio-FS device context:

• DOCA context state change events as described in DOCA Core context state machine and in DOCA DevEmu PCI state machine

• DOCA DevEmu PCI FLR flow

• DOCA DevEmu Virtio reset flow

The DOCA Virtio-FS IO context extends the DOCA Virtio IO context (which extends the DOCA core context). This means that the DOCA Virtio-FS IO context control path is comprised by all the object it extends (i.e., DOCA Context and DOCA DevEmu Virtio IO).

The following events can be triggered by a Virtio-FS IO context:

• DOCA context state change events as described in DOCA Core context state machine

In addition to the control path events, the DOCA DevEmu Virtio-FS IO context also produces IO path events as described in IO path.

IO Path

This section describes the flow for a single virtio-FS request sent by the device driver until its completion.

It is assumed that the user properly configured an event handler for an incoming virtio-FS request as explained in section "Virtio-FS IO Configurations".

It is also assumed that the user is familiar with the virtio-FS specification and has the ability to perform DMA operations to/from the host using DOCA DMA or any other suitable method.

The DOCA virtio-FS flow is illustrated in the following diagram: