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

[Konrad Rzeszutek Wilk]

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.

> 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
> 
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