Re: [PATCH RFC 1/2] qdev: add debug interface to kick/call eventfd

[Eric Blake]

On 1/14/21 6:27 PM, Dongli Zhang wrote:
> The virtio device/driver (e.g., vhost-scsi) may hang due to the lost of IRQ

s/lost/loss/

> or the lost of doorbell register kick, e.g.,

and again

> 
> https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg01711.html
> 
> This patch adds a new debug interface 'DeviceEvent' to DeviceClass to help
> narrow down if the issue is due to lost of irq/kick. So far the new

and again

> interface handles only two events: 'call' and 'kick'. Any device (e.g.,
> e1000e or vhost-scsi) may implement (e.g., via eventfd, MSI-X or legacy
> IRQ).
> 
> The 'call' is to inject irq on purpose by admin for a specific device (e.g.,
> vhost-scsi) from QEMU/host to VM, while the 'kick' is to kick the doorbell
> on purpose by admin at QEMU/host side for a specific device.
> 
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
> ---

> +++ b/qapi/qdev.json
> @@ -124,3 +124,33 @@
>  ##
>  { 'event': 'DEVICE_DELETED',
>    'data': { '*device': 'str', 'path': 'str' } }
> +
> +##
> +# @x-debug-device-event:
> +#
> +# Generate device event for a specific device queue
> +#
> +# @dev: device path
> +#
> +# @event: event (e.g., kick or call) to trigger
> +#
> +# @queue: queue id
> +#
> +# Returns: Nothing on success
> +#
> +# Since: 5.3

The next release is named 6.0, not 5.3.

> +#
> +# Notes: This is used to debug VM driver hang issue. The 'kick' event is to
> +#        send notification to QEMU/vhost while the 'call' event is to
> +#        interrupt VM on purpose.
> +#
> +# Example:
> +#
> +# -> { "execute": "x-debug-device_event",
> +#      "arguments": { "dev": "/machine/peripheral/vscsi0", "event": "kick",
> +#                     "queue": "1" } }

Your example has queue typed as a string...

> +# <- { "return": {} }
> +#
> +##
> +{ 'command': 'x-debug-device-event',
> +  'data': {'dev': 'str', 'event': 'str', 'queue': 'int'} }

...which does not match its actual type as an integer.

event should be an enum type (the finite choice of 'kick' or 'call', and
introspectible if we add new choices in the future) rather than an
open-coded str.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org