linux-kernel - Re: [PATCH net-next V2 0/2] virtio-net: don't busy poll for cvq command

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Date:   Thu, 13 Apr 2023 15:02:47 +0200
From:   Maxime Coquelin <maxime.coquelin@...hat.com>
To:     Jason Wang <jasowang@...hat.com>, mst@...hat.com
Cc:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, alvaro.karsz@...id-run.com,
        eperezma@...hat.com, xuanzhuo@...ux.alibaba.com,
        david.marchand@...hat.com
Subject: Re: [PATCH net-next V2 0/2] virtio-net: don't busy poll for cvq
 command

Hi Jason,

On 4/13/23 08:40, Jason Wang wrote:
> Hi all:
> 
> The code used to busy poll for cvq command which turns out to have
> several side effects:
> 
> 1) infinite poll for buggy devices
> 2) bad interaction with scheduler
> 
> So this series tries to use sleep instead of busy polling. In this
> version, I take a step back: the hardening part is not implemented and
> leave for future investigation. We use to aggree to use interruptible
> sleep but it doesn't work for a general workqueue.
> 
> Please review.

Thanks for working on this.
My DPDK VDUSE RFC missed to set the interrupt, as Xuan Zhou highlighted
it makes the vdpa dev add/del commands to freeze:
[<0>] device_del+0x37/0x3d0
[<0>] device_unregister+0x13/0x60
[<0>] unregister_virtio_device+0x11/0x20
[<0>] device_release_driver_internal+0x193/0x200
[<0>] bus_remove_device+0xbf/0x130
[<0>] device_del+0x174/0x3d0
[<0>] device_unregister+0x13/0x60
[<0>] vdpa_nl_cmd_dev_del_set_doit+0x66/0xe0 [vdpa]
[<0>] genl_family_rcv_msg_doit.isra.0+0xb8/0x100
[<0>] genl_rcv_msg+0x151/0x290
[<0>] netlink_rcv_skb+0x54/0x100
[<0>] genl_rcv+0x24/0x40
[<0>] netlink_unicast+0x217/0x340
[<0>] netlink_sendmsg+0x23e/0x4a0
[<0>] sock_sendmsg+0x8f/0xa0
[<0>] __sys_sendto+0xfc/0x170
[<0>] __x64_sys_sendto+0x20/0x30
[<0>] do_syscall_64+0x59/0x90
[<0>] entry_SYSCALL_64_after_hwframe+0x72/0xdc

Once fixed on DPDK side (you can use my vduse_v1 branch [0] for
testing), it works fine:

Tested-by: Maxime Coquelin <maxime.coquelin@...hat.com>

For the potential missing interrupt with non-compliant devices, I guess
it could be handled with the hardening work as same thing could happen
if the VDUSE application crashed for example.

Regards,
Maxime

[0]:
> Thanks
> 
> Changes since V1:
> - use RTNL to synchronize rx mode worker
> - use completion for simplicity
> - don't try to harden CVQ command
> 
> Changes since RFC:
> 
> - switch to use BAD_RING in virtio_break_device()
> - check virtqueue_is_broken() after being woken up
> - use more_used() instead of virtqueue_get_buf() to allow caller to
>    get buffers afterwards
>    - break the virtio-net device when timeout
>    - get buffer manually since the virtio core check more_used() instead
> 
> Jason Wang (2):
>    virtio-net: convert rx mode setting to use workqueue
>    virtio-net: sleep instead of busy waiting for cvq command
> 
>   drivers/net/virtio_net.c | 76 ++++++++++++++++++++++++++++++++++------
>   1 file changed, 66 insertions(+), 10 deletions(-)
>