[dpdk-dev] vhost-user technical isssues

Linhaifeng haifeng.lin at huawei.com
Thu Nov 13 07:30:31 CET 2014

On 2014/11/12 12:12, Tetsuya Mukawa wrote:
> Hi Xie,
> (2014/11/12 6:37), Xie, Huawei wrote:
>> Hi Tetsuya:
>> There are two major technical issues in my mind for vhost-user implementation.
>> 1) memory region map
>> Vhost-user passes us file fd and offset for each memory region. Unfortunately the mmap offset is "very" wrong. I discovered this issue long time ago, and also found
>> that I couldn't mmap the huge page file even with correct offset(need double check).
>> Just now I find that people reported this issue on Nov 3.
>> [Qemu-devel] [PULL 27/29] vhost-user: fix mmap offset calculation
>> Anyway, I turned to the same idea used in our DPDK vhost-cuse: only use the fd for region(0) to map the  whole file.
>> I think we should use this way temporarily to support qemu-2.1 as it has that bug.
> I agree with you.
> Also we may have an issue about un-mapping file on hugetlbfs of linux.
> When I check munmap(), it seems 'size' need to be aligned by hugepage size.
> (I guess it may be a kernel bug. Might be fixed already.)
> Please add return value checking code for munmap().
> Still munmap() might be failed.
are you munmmap the region 0? region 0 is not need to mmap so not need to munmap too.

I can munmap success with the other regions.

>> 2) what message is the indicator for vhost start/release?
>> Previously  for vhost-cuse, it has SET_BACKEND message.
>> What we should do for vhost-user?
>> SET_VRING_KICK for start?
> I think so.
>> What about for release?
>> Unlike the kernel virtio, the DPDK virtio in guest could be restarted. 
>> Thoughts?
> I guess we need to consider 2 types of restarting.
> One is virtio-net driver restarting, the other is vhost-user backend
> restarting.
> But, so far, it's nice to start to think about virtio-net driver
> restarting first.
> Probably we need to implement a way to let vhost-user backend know
> virtio-net driver is restarted.
> I am not sure what is good way to let vhost-user backend know it.
> But how about followings RFC?
> - When unix domain socket is closed, vhost-user backend should treat it
> as "release".
>  It is useful when QEMU itself is gone suddenly.
> - Also, implementing new ioctl command like VHOST_RESET_BACKEND.
>  This command should be sent from virtio-net device of QEMU when
>  VIRTIO_CONFIG_STATUS_RESET register of virtio-net device is set by
> vrtio-net driver.
>  (Usually this register is set when virtio-net driver is initialized or
> stopped.)
>  It means we need to change QEMU. ;)
>  It seems virtio-net PMD already sets this register when PMD is
> initialized or stopped.
>  So this framework should work, and can let vhost-user backend know
> driver resetting.
>  (And I guess we can say same things for virtio-net kernel driver.)
>  It might be enough to close an unix domain socket, instead of
> implementing new command.
>  But in the case, we may need auto reconnection mechanism.
> - We also need to consider DPDK application is gone suddenly without
> setting reset register.
>  In the case, vhost-user backend cannot know it. Only user (or some kind
> of watchdog
>  applications on guest) knows it.
>  Because of this, user(or app.) should have responsibility to solve this
> situation.
>  To be more precise, user should let vhost-user backend know device
> releasing.
>  If user starts an other DPDK application without solving the issue, the
> new DPDK application may
>  access memory that vhost-user backend is also accessing.
>  I guess user can solve the issue using "dpdk_nic_bind.py".
>  The script can move virtio-net device to kernel virtio-net driver, and
> return it to igb_uio.
>  While those steps, virtio-net device is initialized by virtio-net
> kernel driver.
>  So vhost-user backend can know device releasing.
> Tetsuya
>> -huawei


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