Re: [PATCH 1/2] KVM: async kvm_destroy_vm for vfio devices

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 1/12/23 7:45 AM, Jason Gunthorpe wrote:
> On Wed, Jan 11, 2023 at 08:53:34PM +0000, Sean Christopherson wrote:
>> On Wed, Jan 11, 2023, Jason Gunthorpe wrote:
>>> On Wed, Jan 11, 2023 at 07:54:51PM +0000, Sean Christopherson wrote:
>>>
>>>> Something feels off.  If KVM's refcount is 0, then accessing device->group->kvm
>>>> in vfio_device_open() can't happen unless there's a refcounting bug somewhere.
>>>
>>> The problem is in close, not open.
>>
>> The deadlock problem is, yes.  My point is that if group_lock needs to be taken
>> when nullifying group->kvm during kvm_vfio_destroy(), then there is also a refcounting
>> prolem with respect to open().  If there is no refcounting problem, then nullifying
>> group->kvm during kvm_vfio_destroy() is unnecessary (but again, I doubt this is
>> the case).
> 
> IIRC the drivers are supposed to use one of the refcount not zero
> incrs to counteract this, but I never checked they do..
> 
> Yi is working on a patch to change things so vfio drops the kvm
> pointer when the kvm file closes, not when the reference goes to 0
> to avoid a refcount cycle problem which should also solve that.
> 
>> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
>> index 6e8804fe0095..b3a84d65baa6 100644
>> --- a/drivers/vfio/vfio_main.c
>> +++ b/drivers/vfio/vfio_main.c
>> @@ -772,7 +772,12 @@ static struct file *vfio_device_open(struct vfio_device *device)
>>  		 * reference and release it during close_device.
>>  		 */
>>  		mutex_lock(&device->group->group_lock);
>> -		device->kvm = device->group->kvm;
>> +
>> +		if (device->kvm_ops && device->group->kvm) {
>> +			ret = device->kvm_ops->get_kvm(device->group->kvm);
> 
> At this point I'd rather just use the symbol get stuff like kvm does
> and call the proper functions.
> 

So should I work up a v2 that does symbol gets for kvm_get_kvm_safe and kvm_put_kvm from vfio_main and drop kvm_put_kvm_async?  Or is the patch Yi is working on changing things such that will also address the deadlock issue?
 
If so, something like the following (where vfio_kvm_get symbol gets kvm_get_kvm_safe and vfio_kvm_put symbol gets kvm_put_kvm):

diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 5177bb061b17..a49bf1080f0a 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -361,16 +361,22 @@ static int vfio_device_first_open(struct vfio_device *device,
        if (ret)
                goto err_module_put;
 
+       if (kvm && !vfio_kvm_get(kvm)) {
+               ret = -ENOENT;
+               goto err_unuse_iommu;
+       }
        device->kvm = kvm;
        if (device->ops->open_device) {
                ret = device->ops->open_device(device);
                if (ret)
-                       goto err_unuse_iommu;
+                       goto err_put_kvm;
        }
        return 0;
 
-err_unuse_iommu:
+err_put_kvm:
+       vfio_put_kvm(kvm);
        device->kvm = NULL;
+err_unuse_iommu:
        if (iommufd)
                vfio_iommufd_unbind(device);
        else
@@ -465,6 +471,9 @@ static int vfio_device_fops_release(struct inode *inode, struct file *filep)
 
        vfio_device_group_close(device);
 
+       if (device->open_count == 0 && device->group->kvm)
+               vfio_kvm_put(device->group->kvm);
+
        vfio_device_put_registration(device);
 
        return 0;