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

[Ian Campbell <Ian.Campbell@citrix.com>]

On Tue, 2014-04-08 at 14:53 -0400, 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.

Yes, this is why the ability to have the toolstack save/restore the
secondary vcpu state was added. It's especially important for
checkpointing, but it's relevant to regular migrate as a performance
improvement too.

It's not just stop-machine, IIRC it's a tonne of udev events relating to
cpus off/onlinign etc too and all the userspace activity which that
implies.

Ian.