Subject: [PATCH v2 5/6] arm/arm64: KVM: Turn off vcpus on PSCI shutdown/reboot

From: [email-masked] (Marc Zyngier)
Date: Mon, 08 Dec 2014 13:19:15 +0000

On 08/12/14 12:58, Christoffer Dall wrote:
> On Mon, Dec 08, 2014 at 12:04:53PM +0000, Marc Zyngier wrote:
>> On 03/12/14 21:18, Christoffer Dall wrote:
>>> When a vcpu calls SYSTEM_OFF or SYSTEM_RESET with PSCI v0.2, the vcpus
>>> should really be turned off for the VM adhering to the suggestions in
>>> the PSCI spec, and it's the sane thing to do.
>>>
>>> Also, clarify the behavior and expectations for exits to user space with
>>> the KVM_EXIT_SYSTEM_EVENT case.
>>>
>>> Signed-off-by: Christoffer Dall <[email-masked]>
>>> ---
>>>  Documentation/virtual/kvm/api.txt |  9 +++++++++
>>>  arch/arm/kvm/psci.c               | 19 +++++++++++++++++++
>>>  arch/arm64/include/asm/kvm_host.h |  1 +
>>>  3 files changed, 29 insertions(+)
>>>
>>> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
>>> index 81f1b97..228f9cf 100644
>>> --- a/Documentation/virtual/kvm/api.txt
>>> +++ b/Documentation/virtual/kvm/api.txt
>>> @@ -2957,6 +2957,15 @@ HVC instruction based PSCI call from the vcpu. The 'type' field describes
>>>  the system-level event type. The 'flags' field describes architecture
>>>  specific flags for the system-level event.
>>>  
>>> +Valid values for 'type' are:
>>> +  KVM_SYSTEM_EVENT_SHUTDOWN -- the guest has requested a shutdown of the
>>> +   VM. Userspace is not obliged to honour this, and if it does honour
>>> +   this does not need to destroy the VM synchronously (ie it may call
>>> +   KVM_RUN again before shutdown finally occurs).
>>> +  KVM_SYSTEM_EVENT_RESET -- the guest has requested a reset of the VM.
>>> +   As with SHUTDOWN, userspace can choose to ignore the request, or
>>> +   to schedule the reset to occur in the future and may call KVM_RUN again.
>>> +
>>>  		/* Fix the size of the union. */
>>>  		char padding[256];
>>>  	};
>>> diff --git a/arch/arm/kvm/psci.c b/arch/arm/kvm/psci.c
>>> index 09cf377..ae0bb91 100644
>>> --- a/arch/arm/kvm/psci.c
>>> +++ b/arch/arm/kvm/psci.c
>>> @@ -15,6 +15,7 @@
>>>   * along with this program.  If not, see <http://www.gnu.org/licenses/>.
>>>   */
>>>  
>>> +#include <linux/preempt.h>
>>>  #include <linux/kvm_host.h>
>>>  #include <linux/wait.h>
>>>  
>>> @@ -166,6 +167,24 @@ static unsigned long kvm_psci_vcpu_affinity_info(struct kvm_vcpu *vcpu)
>>>  
>>>  static void kvm_prepare_system_event(struct kvm_vcpu *vcpu, u32 type)
>>>  {
>>> +	int i;
>>> +	struct kvm_vcpu *tmp;
>>> +
>>> +	/*
>>> +	 * The KVM ABI specifies that a system event exit may call KVM_RUN
>>> +	 * again and may perform shutdown/reboot at a later time that when the
>>> +	 * actual request is made.  Since we are implementing PSCI and a
>>> +	 * caller of PSCI reboot and shutdown expects that the system shuts
>>> +	 * down or reboots immediately, let's make sure that VCPUs are not run
>>> +	 * after this call is handled and before the VCPUs have been
>>> +	 * re-initialized.
>>> +	 */
>>> +	kvm_for_each_vcpu(i, tmp, vcpu->kvm)
>>> +		tmp->arch.pause = true;
>>> +	preempt_disable();
>>> +	force_vm_exit(cpu_all_mask);
>>> +	preempt_enable();
>>> +
>>
>> I'm slightly uneasy about this force_vm_exit, as this is something that
>> is directly triggered by the guest. I suppose it is almost impossible to
>> find out which CPUs we're actually using...
>>
> Ah, you mean we should only IPI the CPUs that are actually running a
> VCPU belonging to this VM?
> 
> I guess I could replace it with:
> 
> 	kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
> 		tmp->arch.pause = true;
> 		kvm_vcpu_kick(tmp);
> 	}

Ah, that's even simpler than I thought. Yeah, looks good to me.

> 
> or a slightly more optimized "half-open-coded-kvm_vcpu_kick":
> 
> 	me = get_cpu();
> 	kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
> 		tmp->arch.pause = true;
> 		if (tmp->cpu != me && (unsigned)tmp->cpu < nr_cpu_ids &&
> 		    cpu_online(tmp->cpu)  && kvm_arch_vcpu_should_kick(tmp))
> 			smp_send_reschedule(tmp->cpu);
> 	}
> 
> which should save us waking up vcpu threads that are parked on
> waitqueues.  Not sure it's worth it, maybe it is for 100s of vcpu
> systems?

Probably not worth it at the moment.

> Can we actually replace force_vm_exit() with the more optimized
> open-coded version?  That messes with VMID allocation so it really needs
> a lot of testing though...

VMID reallocation almost never occurs, and that's a system-wide event,
not triggered by a guest. I'd rather not mess with that just yet.

> Preferences?

I think your first version is very nice, provided that it doesn't
introduce any unforeseen regression.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...