Re: [Qemu-devel] [RFC PATCH v0 0/9] Generic cpu-core device

["Andreas Färber" <afaerber@suse.de>]

Am 16.12.2015 um 16:44 schrieb Igor Mammedov:
> On Wed, 16 Dec 2015 16:19:06 +0100
> Andreas Färber <afaerber@suse.de> wrote:
> 
>> Am 10.12.2015 um 07:15 schrieb Bharata B Rao:
>>> CPU hotplug granularity
>>> -----------------------
>>> CPU hotplug will now be done in cpu-core device granularity.
>>
>> Nack.
>>
>>> Are there archs that would need thread level CPU addition ?
>>
>> Yes, s390. And for x86 people called for socket level.
> socket level hotplug would be the last resort if we can't agree
> on thread level one. As it would break existing setups where
> user can hotplug 1 core, and  I'd like to avoid it if it is possible.

We still need to keep cpu-add for backwards compatibility, so I am
discussing solely the new device_add interface. My previous x86 series
went to severe hacks trying to keep cpu-add working with sockets&cores.

Attendees in Seattle said that thread-level hot-plug were dangerous for
Linux guests due to assumptions in the (guest's) scheduler breaking for
any incompletely filled cores or sockets. No one present objected to
doing it on socket level. Bharata(?) had a recent patch to catch such
incompletely filled cores on the initial command line and I think we
should seriously consider doing that even if it breaks some fringe use
case - hot-added incomplete cores or sockets remain to be detected.

Regards,
Andreas

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