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Re: [PATCH RFC 0/2] Add debug interface to kick/call on purpose

From: Daniel P . Berrangé
Subject: Re: [PATCH RFC 0/2] Add debug interface to kick/call on purpose
Date: Fri, 15 Jan 2021 10:27:27 +0000
User-agent: Mutt/1.14.6 (2020-07-11)

On Thu, Jan 14, 2021 at 04:27:28PM -0800, Dongli Zhang wrote:
> The virtio device/driver (e.g., vhost-scsi and indeed any device including
> e1000e) may hang due to the lost of IRQ or the lost of doorbell register
> kick, e.g.,
> https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg01711.html
> The virtio-net was in trouble in above link because the 'kick' was not
> taking effect (missed).
> This RFC 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
> 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.

I'm really not convinced that we want to give admins the direct ability to
poke at internals of devices in a running QEMU. It feels like there is way
too much potential for the admin to make a situation far worse by doing
the wrong thing here, and people dealing with support tickets will have
no idea that the admin has been poking internals of the device and broken
it by doing something wrong.

You pointed to bug that hit where this could conceivably be useful, but
that's a one time issue and should not a common occurrance that justifies
making an official public API to poke at devices forever more IMHO.

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