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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 01/18/2017 06:54 AM, Roger Pau Monné wrote:
> 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.

Yes, I was wrong on both counts --- opt_dom0_max_vcpus_{max|min} use and
offlining at boot time (I was indeed thinking of domU). Sorry about that.

-boris




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