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Re: [PATCH] virtio: add the queue number check

From: Paolo Bonzini
Subject: Re: [PATCH] virtio: add the queue number check
Date: Fri, 31 Jan 2020 17:22:47 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1

I have just found this email... sorry for the delay.

On 10/01/20 07:10, Yang Zhong wrote:
>> No. If virtio-blk works, the bug is in vhost-user-blk; if virtio-blk needs
>> no check in cpu count, vhost-user-blk also doesn't.
>> You need to check first if the bug is in QEMU or the vhost-user-blk server.
>   (1). Seabios issue
>   In init_virtio_blk() function, which set VIRTIO_CONFIG_S_DRIVER_OK
>   status to qemu vhost-user-blk device.
>   // the related code
>   ......
>   vp_set_status(&vdrive->vp, status);
>   ......
>   I think there is no need for seabios to set VIRTIO_CONFIG_S_DRIVER_OK
>   status to qemu vhost-user-blk device.

It does so because it cannot know how it will be used.  It could be used
by the guest boot loader to load a kernel, for example.  SeaBIOS sets
DRIVER_OK because it has loaded a driver for the disk; that's exactly
what DRIVER_OK signals.

>     In fact, this time vhost_user_blk_start almost do nothing because
>     the real guest virtio-blk driver still not started yet. This time,
>     there is only one vq can be used(this vq should be inited in seabios).
>     When the guest virtio-blk driver really start and complet the
>     probe(), the guest virtio-blk driver will set
>     VIRTIO_CONFIG_S_DRIVER_OK to vhost-user-blk device again. This
>     time, this driver will allocate RIGHT queue num according to
>     MIN(vcpu, num_vqs).

Doesn't it first reset the status to 0?

>     (2). DPDK issue
>      DPDK does not know the real queue number used by guest virtio-blk
>      driver and it only know the queue number from vhost-user-blk
>      commond line. Once the guest virtio-blk driver change the queue
>      number according to MIN(vcpu, num_vqs), DPDK still use previous
>      queue number and it think virtio is never ready by
>      virtio_is_ready() function.

What is virtio_is_ready()?  The virtio device should not wait for all
the queues to be set.  A device is ready when it sets DRIVER_OK, and
that's it.

>      or DPDK can get the real queue number by checking if the vring.desc
>      is NON-NULL.

Note that there is no requirement that the driver initializes a
consecutive number of virtqueues.  It is acceptable for it to initialize
virtqueues 0, 1 and 57.  It seems like the bug is in DPDK, possibly more
than one...


>      By the way, vhost SCSI device has the same issue with
>      vhost-user-blk device. 
>      Yang
>> Paolo

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