Re: [Xen-devel] PVH CPU hotplug design document

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jan 18, 2017 at 03:44:19AM -0700, Jan Beulich wrote:
> >>> On 18.01.17 at 11:34, <roger.pau@xxxxxxxxxx> wrote:
> > On Tue, Jan 17, 2017 at 01:50:14PM -0500, Boris Ostrovsky wrote:
> >> On 01/17/2017 12:45 PM, Roger Pau Monné wrote:
> >> > On Tue, Jan 17, 2017 at 10:50:44AM -0500, Boris Ostrovsky wrote:
> >> >> Part of confusion I think is because PV hotplug is not hotplug, really,
> >> >> as far as Linux kernel is concerned.
> >> > Hm, I'm not really sure I'm following, but I think that we could 
> >> > translate this
> >> > Dom0 PV hotplug mechanism to PVH as:
> >> >
> >> >  - Dom0 is provided with up to HVM_MAX_VCPUS local APIC entries in the 
> >> > MADT, and
> >> >    the entries > dom0_max_vcpus are marked as disabled.
> >> >  - Dom0 has HVM_MAX_VCPUS vCPUs ready to be started, either by using the 
> >> > local
> >> >    APIC or an hypercall.
> >> >
> >> > Would that match what's done for classic PV Dom0?
> >> 
> >> To match what we have for PV dom0 I believe you'd provide MADT with
> >> opt_dom0_max_vcpus_max entries and mark all of them enabled.
> >> 
> >> dom0 brings up all opt_dom0_max_vcpus_max VCPUs, and then offlines
> >> (opt_dom0_max_vcpus_min+1)..opt_dom0_max_vcpus_max. See
> >> drivers/xen/cpu_hotplug.c:setup_cpu_watcher(). That's why I said it's
> >> not a hotplug but rather on/off-lining.
> > 
> > But how does Dom0 get the value of opt_dom0_max_vcpus_min? It doesn't seem 
> > to
> > be propagated anywhere from domain_build.
> 
> I'm afraid Boris has given a meaning to that (Xen) command line
> option which it doesn't have. Please see that option's description
> in xen-command-line.markdown. How many vCPU-s should be
> offlined is - iirc - being established by a system boot setting
> inside Dom0.

So, would it be fine to start a PVH Dom0 with as many vCPUs as what's returned
from dom0_max_vcpus, and mark them as enabled in the MADT. That's basically all
we need in order to match current PV Dom0 functionality?

Roger.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel