Re: [Xen-devel] [XEN PATCH 1/2] hvm: Support more than 32 VCPUS when migrating.

["Roger Pau Monné" <roger.pau@citrix.com>]

On 08/04/14 20:53, Konrad Rzeszutek Wilk wrote:
> On Tue, Apr 08, 2014 at 08:18:48PM +0200, Roger Pau Monné wrote:
>> On 08/04/14 19:25, konrad@kernel.org wrote:
>>> From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>>
>>> When we migrate an HVM guest, by default our shared_info can
>>> only hold up to 32 CPUs. As such the hypercall
>>> VCPUOP_register_vcpu_info was introduced which allowed us to
>>> setup per-page areas for VCPUs. This means we can boot PVHVM
>>> guest with more than 32 VCPUs. During migration the per-cpu
>>> structure is allocated fresh by the hypervisor (vcpu_info_mfn
>>> is set to INVALID_MFN) so that the newly migrated guest
>>> can do make the VCPUOP_register_vcpu_info hypercall.
>>>
>>> Unfortunatly we end up triggering this condition:
>>> /* Run this command on yourself or on other offline VCPUS. */
>>>  if ( (v != current) && !test_bit(_VPF_down, &v->pause_flags) )
>>>
>>> which means we are unable to setup the per-cpu VCPU structures
>>> for running vCPUS. The Linux PV code paths make this work by
>>> iterating over every vCPU with:
>>>
>>>  1) is target CPU up (VCPUOP_is_up hypercall?)
>>>  2) if yes, then VCPUOP_down to pause it.
>>>  3) VCPUOP_register_vcpu_info
>>>  4) if it was down, then VCPUOP_up to bring it back up
>>>
>>> But since VCPUOP_down, VCPUOP_is_up, and VCPUOP_up are
>>> not allowed on HVM guests we can't do this. This patch
>>> enables this.
>>
>> Hmmm, this looks like a very convoluted approach to something that could
>> be solved more easily IMHO. What we do on FreeBSD is put all vCPUs into
>> suspension, which means that all vCPUs except vCPU#0 will be in the
>> cpususpend_handler, see:
>>
>> http://svnweb.freebsd.org/base/head/sys/amd64/amd64/mp_machdep.c?revision=263878&view=markup#l1460
> 
> How do you 'suspend' them? If I remember there is a disadvantage of doing
> this as you have to bring all the CPUs "offline". That in Linux means using
> the stop_machine which is pretty big hammer and increases the latency for migration.

In order to suspend them an IPI_SUSPEND is sent to all vCPUs except vCPU#0:

http://fxr.watson.org/fxr/source/kern/subr_smp.c#L289

Which makes all APs call cpususpend_handler, so we know all APs are
stuck in a while loop with interrupts disabled:

http://fxr.watson.org/fxr/source/amd64/amd64/mp_machdep.c#L1459

Then on resume the APs are taken out of the while loop and the first
thing they do before returning from the IPI handler is registering the
new per-cpu vcpu_info area. But I'm not sure this is something that can
be accomplished easily on Linux.

I've tried to local-migrate a FreeBSD PVHVM guest with 33 vCPUs on my
8-way box, and it seems to be working fine :).

Roger.