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SubjectRe: [RFC PATCH 00/22] Enhance VHOST to enable SoC-to-SoC communication
Hi Kishon:

On 2020/9/14 下午3:23, Kishon Vijay Abraham I wrote:
>> Then you need something that is functional equivalent to virtio PCI
>> which is actually the concept of vDPA (e.g vDPA provides alternatives if
>> the queue_sel is hard in the EP implementation).
> Okay, I just tried to compare the 'struct vdpa_config_ops' and 'struct
> vhost_config_ops' ( introduced in [RFC PATCH 03/22] vhost: Add ops for
> the VHOST driver to configure VHOST device).
> struct vdpa_config_ops {
> /* Virtqueue ops */
> int (*set_vq_address)(struct vdpa_device *vdev,
> u16 idx, u64 desc_area, u64 driver_area,
> u64 device_area);
> void (*set_vq_num)(struct vdpa_device *vdev, u16 idx, u32 num);
> void (*kick_vq)(struct vdpa_device *vdev, u16 idx);
> void (*set_vq_cb)(struct vdpa_device *vdev, u16 idx,
> struct vdpa_callback *cb);
> void (*set_vq_ready)(struct vdpa_device *vdev, u16 idx, bool ready);
> bool (*get_vq_ready)(struct vdpa_device *vdev, u16 idx);
> int (*set_vq_state)(struct vdpa_device *vdev, u16 idx,
> const struct vdpa_vq_state *state);
> int (*get_vq_state)(struct vdpa_device *vdev, u16 idx,
> struct vdpa_vq_state *state);
> struct vdpa_notification_area
> (*get_vq_notification)(struct vdpa_device *vdev, u16 idx);
> /* vq irq is not expected to be changed once DRIVER_OK is set */
> int (*get_vq_irq)(struct vdpa_device *vdv, u16 idx);
> /* Device ops */
> u32 (*get_vq_align)(struct vdpa_device *vdev);
> u64 (*get_features)(struct vdpa_device *vdev);
> int (*set_features)(struct vdpa_device *vdev, u64 features);
> void (*set_config_cb)(struct vdpa_device *vdev,
> struct vdpa_callback *cb);
> u16 (*get_vq_num_max)(struct vdpa_device *vdev);
> u32 (*get_device_id)(struct vdpa_device *vdev);
> u32 (*get_vendor_id)(struct vdpa_device *vdev);
> u8 (*get_status)(struct vdpa_device *vdev);
> void (*set_status)(struct vdpa_device *vdev, u8 status);
> void (*get_config)(struct vdpa_device *vdev, unsigned int offset,
> void *buf, unsigned int len);
> void (*set_config)(struct vdpa_device *vdev, unsigned int offset,
> const void *buf, unsigned int len);
> u32 (*get_generation)(struct vdpa_device *vdev);
> /* DMA ops */
> int (*set_map)(struct vdpa_device *vdev, struct vhost_iotlb *iotlb);
> int (*dma_map)(struct vdpa_device *vdev, u64 iova, u64 size,
> u64 pa, u32 perm);
> int (*dma_unmap)(struct vdpa_device *vdev, u64 iova, u64 size);
> /* Free device resources */
> void (*free)(struct vdpa_device *vdev);
> };
> +struct vhost_config_ops {
> + int (*create_vqs)(struct vhost_dev *vdev, unsigned int nvqs,
> + unsigned int num_bufs, struct vhost_virtqueue *vqs[],
> + vhost_vq_callback_t *callbacks[],
> + const char * const names[]);
> + void (*del_vqs)(struct vhost_dev *vdev);
> + int (*write)(struct vhost_dev *vdev, u64 vhost_dst, void *src, int len);
> + int (*read)(struct vhost_dev *vdev, void *dst, u64 vhost_src, int len);
> + int (*set_features)(struct vhost_dev *vdev, u64 device_features);
> + int (*set_status)(struct vhost_dev *vdev, u8 status);
> + u8 (*get_status)(struct vhost_dev *vdev);
> +};
> +
> struct virtio_config_ops
> I think there's some overlap here and some of the ops tries to do the
> same thing.
> I think it differs in (*set_vq_address)() and (*create_vqs)().
> [create_vqs() introduced in struct vhost_config_ops provides
> complimentary functionality to (*find_vqs)() in struct
> virtio_config_ops. It seemingly encapsulates the functionality of
> (*set_vq_address)(), (*set_vq_num)(), (*set_vq_cb)(),..].
> Back to the difference between (*set_vq_address)() and (*create_vqs)(),
> set_vq_address() directly provides the virtqueue address to the vdpa
> device but create_vqs() only provides the parameters of the virtqueue
> (like the number of virtqueues, number of buffers) but does not directly
> provide the address. IMO the backend client drivers (like net or vhost)
> shouldn't/cannot by itself know how to access the vring created on
> virtio front-end. The vdpa device/vhost device should have logic for
> that. That will help the client drivers to work with different types of
> vdpa device/vhost device and can access the vring created by virtio
> irrespective of whether the vring can be accessed via mmio or kernel
> space or user space.
> I think vdpa always works with client drivers in userspace and providing
> userspace address for vring.

Sorry for being unclear. What I meant is not replacing vDPA with the
vhost(bus) you proposed but the possibility of replacing virtio-pci-epf
with vDPA in:

My question is basically for the part of virtio_pci_epf_send_command(),
so it looks to me you have a vendor specific API to replace the
virtio-pci layout of the BAR:

+static int virtio_pci_epf_send_command(struct virtio_pci_device *vp_dev,
+                       u32 command)
+    struct virtio_pci_epf *pci_epf;
+    void __iomem *ioaddr;
+    ktime_t timeout;
+    bool timedout;
+    int ret = 0;
+    u8 status;
+    pci_epf = to_virtio_pci_epf(vp_dev);
+    ioaddr = vp_dev->ioaddr;
+    mutex_lock(&pci_epf->lock);
+    writeb(command, ioaddr + HOST_CMD);
+    timeout = ktime_add_ms(ktime_get(), COMMAND_TIMEOUT);
+    while (1) {
+        timedout = ktime_after(ktime_get(), timeout);
+        status = readb(ioaddr + HOST_CMD_STATUS);

Several questions:

- It's not clear to me how the synchronization is done between the RC
and EP. E.g how and when the value of HOST_CMD_STATUS can be changed. 
If you still want to introduce a new transport, a virtio spec patch
would be helpful for us to understand the device API.
- You have you vendor specific layout (according to
virtio_pci_epb_table()), so I guess you it's better to have a vendor
specific vDPA driver instead
- The advantage of vendor specific vDPA driver is that it can 1) have
less codes 2) support userspace drivers through vhost-vDPA (instead of
inventing new APIs since we can't use vfio-pci here).

>>> "Virtio Over NTB" should anyways be a new transport.
>>>> Does that make any sense?
>>> yeah, in the approach I used the initial features are hard-coded in
>>> vhost-rpmsg (inherent to the rpmsg) but when we have to use adapter
>>> layer (vhost only for accessing virtio ring and use virtio drivers on
>>> both front end and backend), based on the functionality (e.g, rpmsg),
>>> the vhost should be configured with features (to be presented to the
>>> virtio) and that's why additional layer or APIs will be required.
>> A question here, if we go with vhost bus approach, does it mean the
>> virtio device can only be implemented in EP's userspace?
> The vhost bus approach doesn't provide any restriction in where the
> virto backend device should be created. This series creates two types of
> virtio backend device (one for PCIe endpoint and the other for NTB) and
> both these devices are created in kernel.



> Thanks
> Kishon

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