Re: [RFC PATCH v4 17/17] KVM: PPC: Book3S HV: XIVE: introduce a 'release' device operation

[David Gibson <david@gibson.dropbear.id.au>]

[-- Attachment #1: Type: text/plain, Size: 8702 bytes --]

On Mon, Apr 15, 2019 at 03:48:58PM +0200, Cédric Le Goater wrote:
> On 4/15/19 5:32 AM, David Gibson wrote:
> > On Tue, Apr 09, 2019 at 04:13:47PM +0200, Cédric Le Goater wrote:
> >> When the VM boots, the CAS negotiation process determines which
> >> interrupt mode to use and invokes a machine reset. At that time, any
> >> links to the previous KVM interrupt device should be 'destroyed'
> >> before the new chosen one is created.
> >>
> >> To perform the necessary cleanups in KVM, we extend the KVM device
> >> interface with a new 'release' operation which is called when the file
> >> descriptor of the device is closed.
> >>
> >> Such operations are defined for the XICS-on-XIVE and the XIVE native
> >> KVM devices. They clear the vCPU interrupt presenters that could be
> >> attached and then destroy the device.
> >>
> >> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> >> ---
> >>  include/linux/kvm_host.h              |  1 +
> >>  arch/powerpc/kvm/book3s_xive.c        | 50 +++++++++++++++++++++++++--
> >>  arch/powerpc/kvm/book3s_xive_native.c | 23 ++++++++++++
> >>  virt/kvm/kvm_main.c                   | 13 +++++++
> >>  4 files changed, 85 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> >> index 831d963451d8..3b444620d8fc 100644
> >> --- a/include/linux/kvm_host.h
> >> +++ b/include/linux/kvm_host.h
> >> @@ -1246,6 +1246,7 @@ struct kvm_device_ops {
> >>  	long (*ioctl)(struct kvm_device *dev, unsigned int ioctl,
> >>  		      unsigned long arg);
> >>  	int (*mmap)(struct kvm_device *dev, struct vm_area_struct *vma);
> >> +	void (*release)(struct kvm_device *dev);
> >>  };
> >>  
> >>  void kvm_device_get(struct kvm_device *dev);
> >> diff --git a/arch/powerpc/kvm/book3s_xive.c b/arch/powerpc/kvm/book3s_xive.c
> >> index 4d4e1730de84..ba777db849d7 100644
> >> --- a/arch/powerpc/kvm/book3s_xive.c
> >> +++ b/arch/powerpc/kvm/book3s_xive.c
> >> @@ -1100,11 +1100,19 @@ void kvmppc_xive_disable_vcpu_interrupts(struct kvm_vcpu *vcpu)
> >>  void kvmppc_xive_cleanup_vcpu(struct kvm_vcpu *vcpu)
> >>  {
> >>  	struct kvmppc_xive_vcpu *xc = vcpu->arch.xive_vcpu;
> >> -	struct kvmppc_xive *xive = xc->xive;
> >> +	struct kvmppc_xive *xive;
> >>  	int i;
> >>  
> >> +	if (!kvmppc_xics_enabled(vcpu))
> >> +		return;
> >> +
> >> +	if (!xc)
> >> +		return;
> >> +
> >>  	pr_devel("cleanup_vcpu(cpu=%d)\n", xc->server_num);
> >>  
> >> +	xive = xc->xive;
> >> +
> >>  	/* Ensure no interrupt is still routed to that VP */
> >>  	xc->valid = false;
> >>  	kvmppc_xive_disable_vcpu_interrupts(vcpu);
> >> @@ -1141,6 +1149,10 @@ void kvmppc_xive_cleanup_vcpu(struct kvm_vcpu *vcpu)
> >>  	}
> >>  	/* Free the VP */
> >>  	kfree(xc);
> >> +
> >> +	/* Cleanup the vcpu */
> >> +	vcpu->arch.irq_type = KVMPPC_IRQ_DEFAULT;
> >> +	vcpu->arch.xive_vcpu = NULL;
> >>  }
> >>  
> >>  int kvmppc_xive_connect_vcpu(struct kvm_device *dev,
> >> @@ -1158,7 +1170,7 @@ int kvmppc_xive_connect_vcpu(struct kvm_device *dev,
> >>  	}
> >>  	if (xive->kvm != vcpu->kvm)
> >>  		return -EPERM;
> >> -	if (vcpu->arch.irq_type)
> >> +	if (vcpu->arch.irq_type != KVMPPC_IRQ_DEFAULT)
> >>  		return -EBUSY;
> >>  	if (kvmppc_xive_find_server(vcpu->kvm, cpu)) {
> >>  		pr_devel("Duplicate !\n");
> >> @@ -1855,6 +1867,39 @@ static void kvmppc_xive_free(struct kvm_device *dev)
> >>  	kfree(dev);
> >>  }
> >>  
> >> +static void kvmppc_xive_release(struct kvm_device *dev)
> >> +{
> >> +	struct kvmppc_xive *xive = dev->private;
> >> +	struct kvm *kvm = xive->kvm;
> >> +	struct kvm_vcpu *vcpu;
> >> +	int i;
> >> +
> >> +	pr_devel("Releasing xive device\n");
> >> +
> >> +	/*
> >> +	 * When releasing the KVM device fd, the vCPUs can still be
> >> +	 * running and we should clean up the vCPU interrupt
> >> +	 * presenters first.
> >> +	 */
> >> +	if (atomic_read(&kvm->online_vcpus) != 0) {
> > 
> > What prevents online_vcpus from becoming non-zero after this test, but
> > before the kvmppc_xive_free()?
> 
> I am not sure what you mean. kvmppc_xive_free() is gone with this patch. 
> It has been replaced by kvmppc_xive_release().
> 
> > Is the test actually necessary?  The operations below should be safe
> > even if there are no online cpus, yes?
> 
> ah, yes. kvm_for_each_vcpu() should be safe to use anyhow.
> 
> >> +		/*
> >> +		 * call kick_all_cpus_sync() to ensure that all CPUs
> >> +		 * have executed any pending interrupts
> >> +		 */
> >> +		if (is_kvmppc_hv_enabled(kvm))
> >> +			kick_all_cpus_sync();>> +		/*
> >> +		 * TODO: There is still a race window with the early
> >> +		 * checks in kvmppc_native_connect_vcpu()
> >> +		 */
> > 
> > That's... not reassuring.  What are the consequences of that race, 
> 
> a bogus ->xive pointer under the XIVE vCPU
> 
> > and what do you plan to do about it?
> 
> I don't think this is true any more with the release operation
> which will be called by the last user of the device file.

Ok, so the comment needs updating.

> Anyhow, xc->xive does not seem very useful (just like xc->valid) 
> We should try to use only vcpu->kvm->arch.xive instead.
> 
> I will propose some preliminary cleanups before introducing the
> new release operation.
> 
> >> +		kvm_for_each_vcpu(i, vcpu, kvm)
> >> +			kvmppc_xive_cleanup_vcpu(vcpu);
> >> +	}
> >> +
> >> +	kvmppc_xive_free(dev);
> >> +}
> >> +
> >>  struct kvmppc_xive *kvmppc_xive_get_device(struct kvm *kvm, u32 type)
> >>  {
> >>  	struct kvmppc_xive *xive;
> >> @@ -2043,6 +2088,7 @@ struct kvm_device_ops kvm_xive_ops = {
> >>  	.name = "kvm-xive",
> >>  	.create = kvmppc_xive_create,
> >>  	.init = kvmppc_xive_init,
> >> +	.release = kvmppc_xive_release,
> >>  	.destroy = kvmppc_xive_free,
> >>  	.set_attr = xive_set_attr,
> >>  	.get_attr = xive_get_attr,
> >> diff --git a/arch/powerpc/kvm/book3s_xive_native.c b/arch/powerpc/kvm/book3s_xive_native.c
> >> index 092db0efe628..629da7bf2a89 100644
> >> --- a/arch/powerpc/kvm/book3s_xive_native.c
> >> +++ b/arch/powerpc/kvm/book3s_xive_native.c
> >> @@ -996,6 +996,28 @@ static void kvmppc_xive_native_free(struct kvm_device *dev)
> >>  	kfree(dev);
> >>  }
> >>  
> >> +static void kvmppc_xive_native_release(struct kvm_device *dev)
> >> +{
> >> +	struct kvmppc_xive *xive = dev->private;
> >> +	struct kvm *kvm = xive->kvm;
> >> +	struct kvm_vcpu *vcpu;
> >> +	int i;
> >> +
> >> +	pr_devel("Releasing xive native device\n");
> >> +
> >> +	/*
> >> +	 * When releasing the KVM device fd, the vCPUs can still be
> >> +	 * running and we should clean up the vCPU interrupt
> >> +	 * presenters first.
> >> +	 */
> >> +	if (atomic_read(&kvm->online_vcpus) != 0) {
> > 
> > Likewise here.
> > 
> >> +		kvm_for_each_vcpu(i, vcpu, kvm)
> >> +			kvmppc_xive_native_cleanup_vcpu(vcpu);
> >> +	}
> >> +
> >> +	kvmppc_xive_native_free(dev);
> >> +}
> >> +
> >>  static int kvmppc_xive_native_create(struct kvm_device *dev, u32 type)
> >>  {
> >>  	struct kvmppc_xive *xive;
> >> @@ -1187,6 +1209,7 @@ struct kvm_device_ops kvm_xive_native_ops = {
> >>  	.name = "kvm-xive-native",
> >>  	.create = kvmppc_xive_native_create,
> >>  	.init = kvmppc_xive_native_init,
> >> +	.release = kvmppc_xive_native_release,
> >>  	.destroy = kvmppc_xive_native_free,
> >>  	.set_attr = kvmppc_xive_native_set_attr,
> >>  	.get_attr = kvmppc_xive_native_get_attr,
> >> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> >> index ea2018ae1cd7..ea2619d5ca98 100644
> >> --- a/virt/kvm/kvm_main.c
> >> +++ b/virt/kvm/kvm_main.c
> >> @@ -2938,6 +2938,19 @@ static int kvm_device_release(struct inode *inode, struct file *filp)
> >>  	struct kvm_device *dev = filp->private_data;
> >>  	struct kvm *kvm = dev->kvm;
> >>  
> >> +	if (!dev)
> >> +		return -ENODEV;
> >> +
> >> +	if (dev->kvm != kvm)
> >> +		return -EPERM;
> >> +
> >> +	if (dev->ops->release) {
> >> +		mutex_lock(&kvm->lock);
> >> +		list_del(&dev->vm_node);
> >> +		dev->ops->release(dev);
> >> +		mutex_unlock(&kvm->lock);
> >> +	}
> >> +
> > 
> > Wasn't there a big comment that explained that release replaced
> > destroy somewhere?
> 
> Yes. I did add a comment in the "V5 errata" series. 
> 
> I should be sending a v6 this week, to clarify all these attempts 
> to solve the device switching.
> 
> Thanks,
> 
> C. 
> 
> > 
> >>  	kvm_put_kvm(kvm);
> >>  	return 0;
> >>  }
> > 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]