linux-kernel - Re: [RFC v2 4/4] vduse: Add new ioctl VDUSE_GET_RECONNECT

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Message-ID: <6c4cd924-0d44-582e-13a4-791f38d10fe8@redhat.com>
Date:   Fri, 29 Sep 2023 11:08:08 +0200
From:   Maxime Coquelin <maxime.coquelin@...hat.com>
To:     Jason Wang <jasowang@...hat.com>, Cindy Lu <lulu@...hat.com>
Cc:     mst@...hat.com, xieyongji@...edance.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [RFC v2 4/4] vduse: Add new ioctl VDUSE_GET_RECONNECT_INFO



On 9/25/23 04:57, Jason Wang wrote:
> On Thu, Sep 21, 2023 at 10:07 PM Cindy Lu <lulu@...hat.com> wrote:
>>
>> On Mon, Sep 18, 2023 at 4:49 PM Jason Wang <jasowang@...hat.com> wrote:
>>>
>>> On Tue, Sep 12, 2023 at 11:01 AM Cindy Lu <lulu@...hat.com> wrote:
>>>>
>>>> In VDUSE_GET_RECONNECT_INFO, the Userspace App can get the map size
>>>> and The number of mapping memory pages from the kernel. The userspace
>>>> App can use this information to map the pages.
>>>>
>>>> Signed-off-by: Cindy Lu <lulu@...hat.com>
>>>> ---
>>>>   drivers/vdpa/vdpa_user/vduse_dev.c | 15 +++++++++++++++
>>>>   include/uapi/linux/vduse.h         | 15 +++++++++++++++
>>>>   2 files changed, 30 insertions(+)
>>>>
>>>> diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
>>>> index 680b23dbdde2..c99f99892b5c 100644
>>>> --- a/drivers/vdpa/vdpa_user/vduse_dev.c
>>>> +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
>>>> @@ -1368,6 +1368,21 @@ static long vduse_dev_ioctl(struct file *file, unsigned int cmd,
>>>>                  ret = 0;
>>>>                  break;
>>>>          }
>>>> +       case VDUSE_GET_RECONNECT_INFO: {
>>>> +               struct vduse_reconnect_mmap_info info;
>>>> +
>>>> +               ret = -EFAULT;
>>>> +               if (copy_from_user(&info, argp, sizeof(info)))
>>>> +                       break;
>>>> +
>>>> +               info.size = PAGE_SIZE;
>>>> +               info.max_index = dev->vq_num + 1;
>>>> +
>>>> +               if (copy_to_user(argp, &info, sizeof(info)))
>>>> +                       break;
>>>> +               ret = 0;
>>>> +               break;
>>>> +       }
>>>>          default:
>>>>                  ret = -ENOIOCTLCMD;
>>>>                  break;
>>>> diff --git a/include/uapi/linux/vduse.h b/include/uapi/linux/vduse.h
>>>> index d585425803fd..ce55e34f63d7 100644
>>>> --- a/include/uapi/linux/vduse.h
>>>> +++ b/include/uapi/linux/vduse.h
>>>> @@ -356,4 +356,19 @@ struct vhost_reconnect_vring {
>>>>          _Bool avail_wrap_counter;
>>>>   };
>>>>
>>>> +/**
>>>> + * struct vduse_reconnect_mmap_info
>>>> + * @size: mapping memory size, always page_size here
>>>> + * @max_index: the number of pages allocated in kernel,just
>>>> + * use for check
>>>> + */
>>>> +
>>>> +struct vduse_reconnect_mmap_info {
>>>> +       __u32 size;
>>>> +       __u32 max_index;
>>>> +};
>>>
>>> One thing I didn't understand is that, aren't the things we used to
>>> store connection info belong to uAPI? If not, how can we make sure the
>>> connections work across different vendors/implementations. If yes,
>>> where?
>>>
>>> Thanks
>>>
>> The process for this reconnecttion  is
>> A.The first-time connection
>> 1> The userland app checks if the device exists
>> 2>  use the ioctl to create the vduse device
>> 3> Mapping the kernel page to userland and save the
>> App-version/features/other information to this page
>> 4>  if the Userland app needs to exit, then the Userland app will only
>> unmap the page and then exit
>>
>> B, the re-connection
>> 1> the userland app finds the device is existing
>> 2> Mapping the kernel page to userland
>> 3> check if the information in shared memory is satisfied to
>> reconnect,if ok then continue to reconnect
>> 4> continue working
>>
>>   For now these information are all from userland,So here the page will
>> be maintained by the userland App
>> in the previous code we only saved the api-version by uAPI .  if  we
>> need to support reconnection maybe we need to add 2 new uAPI for this,
>> one of the uAPI is to save the reconnect  information and another is
>> to get the information
>>
>> maybe something like
>>
>> struct vhost_reconnect_data {
>> uint32_t version;
>> uint64_t features;
>> uint8_t status;
>> struct virtio_net_config config;
>> uint32_t nr_vrings;
>> };
> 
> Probably, then we can make sure the re-connection works across
> different vduse-daemon implementations.

+1, we need to have this defined in the uAPI to support interoperability
across different VDUSE userspace implementations.

> 
>>
>> #define VDUSE_GET_RECONNECT_INFO _IOR (VDUSE_BASE, 0x1c, struct
>> vhost_reconnect_data)
>>
>> #define VDUSE_SET_RECONNECT_INFO  _IOWR(VDUSE_BASE, 0x1d, struct
>> vhost_reconnect_data)
> 
> Not sure I get this, but the idea is to map those pages to user space,
> any reason we need this uAPI?

It should not be necessary if the mmapped layout is properly defined.

Thanks,
Maxime

> Thanks
> 
>>
>> Thanks
>> Cindy
>>
>>
>>
>>
>>>> +
>>>> +#define VDUSE_GET_RECONNECT_INFO \
>>>> +       _IOWR(VDUSE_BASE, 0x1b, struct vduse_reconnect_mmap_info)
>>>> +
>>>>   #endif /* _UAPI_VDUSE_H_ */
>>>> --
>>>> 2.34.3
>>>>
>>>
>>
>