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

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Apr 09, 2014 at 04:38:37PM +0100, David Vrabel wrote:
> On 09/04/14 16:34, Konrad Rzeszutek Wilk wrote:
> > On Wed, Apr 09, 2014 at 09:37:01AM +0200, Roger Pau Monné wrote:
> >> 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.
> > 
> > That is a bit of what the 'stop_machine' would do. It puts all of the
> > CPUs in whatever function you want. But I am not sure of the latency impact - as
> > in what if the migration takes longer and all of the CPUs sit there spinning.
> > Another variant of that is the 'smp_call_function'.
> 
> I tested stop_machine() on all CPUs during suspend once and it was
> awful:  100s of ms of additional downtime.

Yikes.
> 
> Perhaps a hand-rolled IPI-and-park-in-handler would be quicker the full
> stop_machine().

But that is clearly a bigger patch than this little bug-fix.

Do you want to just take this patch as is and then later on I can work on
prototyping the 'IPI-and-park-in-handler'?