by Jason Wang

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Subject: Re: [RFC PATCH 00/22] Enhance VHOST to enable SoC-to-SoC communication

On 2020/9/16 下午7:47, Kishon Vijay Abraham I wrote:
> Hi Jason,
>
> On 16/09/20 8:40 am, Jason Wang wrote:
>> On 2020/9/15 下午11:47, Kishon Vijay Abraham I wrote:
>>> Hi Jason,
>>>
>>> On 15/09/20 1:48 pm, Jason Wang wrote:
>>>> 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:
>>> Okay, so the virtio back-end still use vhost and front end should use
>>> vDPA. I see. So the host side PCI driver for EPF should populate
>>> vdpa_config_ops and invoke vdpa_register_device().
>>
>> Yes.
>>
>>
>>>> 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:
>>> Even when we use vDPA, we have to use some sort of
>>> virtio_pci_epf_send_command() to communicate with virtio backend right?
>>
>> Right.
>>
>>
>>> Right, the layout is slightly different from the standard layout.
>>>
>>> This is the layout
>>> struct epf_vhost_reg_queue {
>>>          u8 cmd;
>>>          u8 cmd_status;
>>>          u16 status;
>>>          u16 num_buffers;
>>>          u16 msix_vector;
>>>          u64 queue_addr;
>>
>> What's the meaning of queue_addr here?
> Using queue_addr, the virtio front-end communicates the address of the
> allocated memory for virtqueue to the virtio back-end.
>> Does not mean the device expects a contiguous memory for avail/desc/used
>> ring?
> It's contiguous memory. Isn't this similar to other virtio transport
> (both PCI legacy and modern interface)?.

That's only for legacy device, for modern device we don't have such
restriction.

>>
>>> } __packed;
>>>
>>> struct epf_vhost_reg {
>>>          u64 host_features;
>>>          u64 guest_features;
>>>          u16 msix_config;
>>>          u16 num_queues;
>>>          u8 device_status;
>>>          u8 config_generation;
>>>          u32 isr;
>>>          u8 cmd;
>>>          u8 cmd_status;
>>>          struct epf_vhost_reg_queue vq[MAX_VQS];
>>> } __packed;
>>>> +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.
>>> The HOST_CMD (commands sent to the EP) is serialized by using mutex.
>>> Once the EP reads the command, it resets the value in HOST_CMD. So
>>> HOST_CMD is less likely an issue.
>>
>> Here's my understanding of the protocol:
>>
>> 1) RC write to HOST_CMD
>> 2) RC wait for HOST_CMD_STATUS to be HOST_CMD_STATUS_OKAY
> That's right!
>> It looks to me what EP should do is
>>
>> 1) EP reset HOST_CMD after reading new command
> That's right! It does.
>> And it looks to me EP should also reset HOST_CMD_STATUS here?
> yeah, that would require RC to send another command to reset the status.
> Didn't see it required in the normal scenario but good to add this.
>> (I thought there should be patch to handle stuffs like this but I didn't
>> find it in this series)
> This is added in [RFC PATCH 19/22] PCI: endpoint: Add EP function driver
> to provide VHOST interface
>
> pci_epf_vhost_cmd_handler() gets commands from RC using "reg->cmd;". On
> the EP side, it is local memory access (mapped to BAR memory exposed to
> the host) and hence accessed using structure member access.

Thanks for the pointer, will have a look at and I think this part need
to be carefully designed and the key to the success of the epf transport.