[libvirt] [PATCH v1 1/3] adding unplug_timeout QEMU conf

[Daniel P. Berrangé]

On Mon, Aug 19, 2019 at 11:16:24AM +0200, Ján Tomko wrote:
> On Sun, Aug 18, 2019 at 06:45:29PM -0300, Daniel Henrique Barboza wrote:
> > For some architectures and setups, device removal can take
> > longer than the default 5 seconds. This results in commands
> > such as 'virsh setvcpus' to fire timeout messages even if
> > the actual operation happened in the guest, causing confusion
> > for the user.
> > 
> 
> The commit that introduced this error message:
>    commit e3229f6e4461cd1721dc68a32e16ab1718ae716e
>        qemu: hotplug: Add support for VCPU unplug
> 
> specifically says that we treat this differently than regular device
> detach:
> 
>    As the new code is using device_del all the implications of using it
>    are present. Contrary to the device deletion code, the vcpu deletion
>    code fails if the unplug request is not executed in time.
> 
> Technically, we already did execute the unplug request so we lie to the
> user saying "operation failed".
> 
> Maybe we can revisit the decision? [cc-ing pkrempa who added this]
> 
> > This patch adds a new qemu.conf parameter called 'unplug_timeout'
> > to handle these cases. If left unset, the current default
> > timeout is used. To avoid user 'experimentation' with small
> > timeouts, the current timeout is also the minimal value
> > allowed.
> > 
> 
> The reason for this timeout is that we originally promised something
> that we cannot deliver - a synchronous device detach API, while the
> operation itself is asynchronous. I'm not a fan of exposing it and
> making it configurable.

I'm especially *not* a fan because the commit messages says this is
a problem on certain architectures. Since we know what those arches
are, we should use a larger timeout for those arches out of the box.
Requiring admin to set a config param to fix the architectures is
super unpleasant out of the box experiance.


Regards,
Daniel
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