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

[Igor Mammedov]

On Wed, 16 Dec 2015 16:57:54 +0100
Andreas Färber <address@hidden> wrote:

> Am 16.12.2015 um 16:44 schrieb Igor Mammedov:
> > On Wed, 16 Dec 2015 16:19:06 +0100
> > Andreas Färber <address@hidden> 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.
if possible, it would be better to make cpu-add to use device_add
internally.

> 
> 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
There is not such thing as cpu hotplug at socket level in x86 linux so far.
CPUs are plugged at logical(thread) cpu level, one at a time.
And ACPI spec does the same (describes logical CPUs) and hotplug
notification in guest handled per one logical cpu at a time.

> 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.
interesting question, what MADT/SRAT tables would look like on machine
with Intel CPU when hyperthreading is disabled in BIOS? 
 
> 
> Regards,
> Andreas
>