Re: [PATCH v3] s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated

[Halil Pasic <pasic@linux.ibm.com>]

On Tue, 15 Dec 2020 19:10:20 +0100
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> 
> 
> On 15.12.20 11:57, Halil Pasic wrote:
> > On Mon, 14 Dec 2020 11:56:17 -0500
> > Tony Krowiak <akrowiak@linux.ibm.com> wrote:
> > 
> >> The vfio_ap device driver registers a group notifier with VFIO when the
> >> file descriptor for a VFIO mediated device for a KVM guest is opened to
> >> receive notification that the KVM pointer is set (VFIO_GROUP_NOTIFY_SET_KVM
> >> event). When the KVM pointer is set, the vfio_ap driver takes the
> >> following actions:
> >> 1. Stashes the KVM pointer in the vfio_ap_mdev struct that holds the state
> >>    of the mediated device.
> >> 2. Calls the kvm_get_kvm() function to increment its reference counter.
> >> 3. Sets the function pointer to the function that handles interception of
> >>    the instruction that enables/disables interrupt processing.
> >> 4. Sets the masks in the KVM guest's CRYCB to pass AP resources through to
> >>    the guest.
> >>
> >> In order to avoid memory leaks, when the notifier is called to receive
> >> notification that the KVM pointer has been set to NULL, the vfio_ap device
> >> driver should reverse the actions taken when the KVM pointer was set.
> >>
> >> Fixes: 258287c994de ("s390: vfio-ap: implement mediated device open callback")
> >> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
> >> ---
> >>  drivers/s390/crypto/vfio_ap_ops.c | 29 ++++++++++++++++++++---------
> >>  1 file changed, 20 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> >> index e0bde8518745..cd22e85588e1 100644
> >> --- a/drivers/s390/crypto/vfio_ap_ops.c
> >> +++ b/drivers/s390/crypto/vfio_ap_ops.c
> >> @@ -1037,8 +1037,6 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
> >>  {
> >>  	struct ap_matrix_mdev *m;
> >>
> >> -	mutex_lock(&matrix_dev->lock);
> >> -
> >>  	list_for_each_entry(m, &matrix_dev->mdev_list, node) {
> >>  		if ((m != matrix_mdev) && (m->kvm == kvm)) {
> >>  			mutex_unlock(&matrix_dev->lock);
> >> @@ -1049,7 +1047,6 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
> >>  	matrix_mdev->kvm = kvm;
> >>  	kvm_get_kvm(kvm);
> >>  	kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
> >> -	mutex_unlock(&matrix_dev->lock);
> >>
> >>  	return 0;
> >>  }
> >> @@ -1083,35 +1080,49 @@ static int vfio_ap_mdev_iommu_notifier(struct notifier_block *nb,
> >>  	return NOTIFY_DONE;
> >>  }
> >>
> >> +static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
> >> +{
> >> +	kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
> >> +	matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> > 
> > 
> > This patch LGTM. The only concern I have with it is whether a
> > different cpu is guaranteed to observe the above assignment as
> > an atomic operation. I think we didn't finish this discussion
> > at v1, or did we?
> 
> You mean just this assigment:
> >> +	matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> should either have the old or the new value, but not halve zero halve old?
>

Yes that is the assignment I was referring to. Old value will work as well because
kvm holds a reference to this module while in the pqap_hook.
 
> Normally this should be ok (and I would consider this a compiler bug if
> this is split into 2 32 bit zeroes) But if you really want to be sure then we
> can use WRITE_ONCE.

Just my curiosity: what would make this a bug? Is it the s390 elf ABI,
or some gcc feature, or even the C standard? Also how exactly would
WRITE_ONCE, also access via volatile help in this particular situation?

I agree, if the member is properly aligned, (which it is),
normally/probably we are fine on s390x (which is also a given). 

> I think we take this via the s390 tree? I can add the WRITE_ONCE when applying?

Yes that works fine with me.

Reviewed-by: Halil Pasic <pasic@linux.ibm.com>