Re: [PATCH] xen/pvhvm: Support more than 32 VCPUs when migrating (v3).

[Konrad Rzeszutek Wilk]

On Fri, Jul 10, 2015 at 02:57:51PM -0400, Konrad Rzeszutek Wilk wrote:
> On Fri, Jul 10, 2015 at 02:37:46PM -0400, Konrad Rzeszutek Wilk wrote:
> > When Xen migrates an HVM guest, by default its 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 freshly by the hypervisor (vcpu_info_mfn
> > is set to INVALID_MFN) so that the newly migrated guest
> > can make an VCPUOP_register_vcpu_info hypercall.
> > 
> > Unfortunatly we end up triggering this condition in Xen:
> > /* 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. However with the
> > Xen git commit f80c5623a126afc31e6bb9382268d579f0324a7a
> > ("xen/x86: allow HVM guests to use hypercalls to bring up vCPUs"")
> 
> <sigh> I was in my local tree which was Roger's 'hvm_without_dm_v3'
> looking at patches and spotted this - and thought it was already in!
> 
> Sorry about this patch - and please ignore it until the VCPU_op*
> can be used by HVM guests.

The corresponding patch is in Xen tree (192df6f9122ddebc21d0a632c10da3453aeee1c2)

Could folks take a look at the patch pls?

Without it you can't migrate an Linux guest with more than 32 vCPUs.

> 
> > we can do this. As such first check if VCPUOP_is_up is actually
> > possible before trying this dance.
> > 
> > As most of this dance code is done already in 'xen_setup_vcpu'
> > lets make it callable on both PV and HVM. This means moving one
> > of the checks out to 'xen_setup_runstate_info'.
> > 
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > ---
> >  arch/x86/xen/enlighten.c | 23 +++++++++++++++++------
> >  arch/x86/xen/suspend.c   |  7 +------
> >  arch/x86/xen/time.c      |  3 +++
> >  3 files changed, 21 insertions(+), 12 deletions(-)
> > 
> > diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> > index 46957ea..187dec6 100644
> > --- a/arch/x86/xen/enlighten.c
> > +++ b/arch/x86/xen/enlighten.c
> > @@ -238,12 +238,23 @@ static void xen_vcpu_setup(int cpu)
> >  void xen_vcpu_restore(void)
> >  {
> >  	int cpu;
> > +	bool vcpuops = true;
> > +	const struct cpumask *mask;
> >  
> > -	for_each_possible_cpu(cpu) {
> > +	mask = xen_pv_domain() ? cpu_possible_mask : cpu_online_mask;
> > +
> > +	/* Only Xen 4.5 and higher supports this. */
> > +	if (HYPERVISOR_vcpu_op(VCPUOP_is_up, smp_processor_id(), NULL) == -ENOSYS)
> > +		vcpuops = false;
> > +
> > +	for_each_cpu(cpu, mask) {
> >  		bool other_cpu = (cpu != smp_processor_id());
> > -		bool is_up = HYPERVISOR_vcpu_op(VCPUOP_is_up, cpu, NULL);
> > +		bool is_up = false;
> >  
> > -		if (other_cpu && is_up &&
> > +		if (vcpuops)
> > +			is_up = HYPERVISOR_vcpu_op(VCPUOP_is_up, cpu, NULL);
> > +
> > +		if (vcpuops && other_cpu && is_up &&
> >  		    HYPERVISOR_vcpu_op(VCPUOP_down, cpu, NULL))
> >  			BUG();
> >  
> > @@ -252,7 +263,7 @@ void xen_vcpu_restore(void)
> >  		if (have_vcpu_info_placement)
> >  			xen_vcpu_setup(cpu);
> >  
> > -		if (other_cpu && is_up &&
> > +		if (vcpuops && other_cpu && is_up &&
> >  		    HYPERVISOR_vcpu_op(VCPUOP_up, cpu, NULL))
> >  			BUG();
> >  	}
> > @@ -1704,8 +1715,8 @@ void __ref xen_hvm_init_shared_info(void)
> >  	 * in that case multiple vcpus might be online. */
> >  	for_each_online_cpu(cpu) {
> >  		/* Leave it to be NULL. */
> > -		if (cpu >= MAX_VIRT_CPUS)
> > -			continue;
> > +		if (cpu >= MAX_VIRT_CPUS && cpu <= NR_CPUS)
> > +			per_cpu(xen_vcpu, cpu) = NULL; /* Triggers xen_vcpu_setup.*/
> >  		per_cpu(xen_vcpu, cpu) = &HYPERVISOR_shared_info->vcpu_info[cpu];
> >  	}
> >  }
> > diff --git a/arch/x86/xen/suspend.c b/arch/x86/xen/suspend.c
> > index 53b4c08..cd66397 100644
> > --- a/arch/x86/xen/suspend.c
> > +++ b/arch/x86/xen/suspend.c
> > @@ -31,15 +31,10 @@ static void xen_pv_pre_suspend(void)
> >  static void xen_hvm_post_suspend(int suspend_cancelled)
> >  {
> >  #ifdef CONFIG_XEN_PVHVM
> > -	int cpu;
> >  	xen_hvm_init_shared_info();
> >  	xen_callback_vector();
> >  	xen_unplug_emulated_devices();
> > -	if (xen_feature(XENFEAT_hvm_safe_pvclock)) {
> > -		for_each_online_cpu(cpu) {
> > -			xen_setup_runstate_info(cpu);
> > -		}
> > -	}
> > +	xen_vcpu_restore();
> >  #endif
> >  }
> >  
> > diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> > index 55da33b..6882d0c 100644
> > --- a/arch/x86/xen/time.c
> > +++ b/arch/x86/xen/time.c
> > @@ -105,6 +105,9 @@ void xen_setup_runstate_info(int cpu)
> >  {
> >  	struct vcpu_register_runstate_memory_area area;
> >  
> > +	if (xen_hvm_domain() && !(xen_feature(XENFEAT_hvm_safe_pvclock)))
> > +		return;
> > +
> >  	area.addr.v = &per_cpu(xen_runstate, cpu);
> >  
> >  	if (HYPERVISOR_vcpu_op(VCPUOP_register_runstate_memory_area,
> > -- 
> > 2.1.0
> >