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

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Date:   Wed, 16 Dec 2020 23:00:31 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Tony Krowiak <akrowiak@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        cohuck@...hat.com, kwankhede@...dia.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, pasic@...ux.vnet.ibm.com
Subject: Re: [PATCH v3] s390/vfio-ap: clean up vfio_ap resources when KVM
 pointer invalidated

On Wed, 16 Dec 2020 17:05:24 +0100
Christian Borntraeger <borntraeger@...ibm.com> wrote:

> 
> 
> On 16.12.20 10:58, Christian Borntraeger wrote:
> > On 16.12.20 02:21, Halil Pasic wrote:
> >> On Tue, 15 Dec 2020 19:10:20 +0100
> >> Christian Borntraeger <borntraeger@...ibm.com> wrote:
> >>
> >>>
> >>>
> >>> On 15.12.20 11:57, Halil Pasic wrote:
> >>>> On Mon, 14 Dec 2020 11:56:17 -0500
> >>>> Tony Krowiak <akrowiak@...ux.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@...ux.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 "(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 think its a tricky things and not strictly guaranteed, but there is a lot
> > of code that relies on the atomicity of word sizes. see for example the discussion
> > here
> > https://lore.kernel.org/lkml/CAHk-=wgC4+kV9AiLokw7cPP429rKCU+vjA8cWAfyOjC3MtqC4A@mail.gmail.com/
> > 
> > WRITE_ONCE will not change the guarantees a lot, but it is mostly a documentation
> > that we assume atomic access here.
> 
> After looking again at the code, I think I have to correct myself.
> WRITE_ONCE does not look necessary.
> 

Yes, volatile access is not about atomic access. Whether a volatile
access here is a good idea or not, is a different question.

> 
> Another thing, though:
> Shouldnt we also replace this code
> 
> [...]
> static void vfio_ap_mdev_release(struct mdev_device *mdev)
> {
>         struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
> 
>         mutex_lock(&matrix_dev->lock);
>         if (matrix_mdev->kvm) {
> --->          kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
> --->          matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
> --->          vfio_ap_mdev_reset_queues(mdev);
> --->          kvm_put_kvm(matrix_mdev->kvm);
> --->          matrix_mdev->kvm = NULL;
> [...]
> 
> with vfio_ap_mdev_unset_kvm ?

Yes, we had something like this in v2 already. Would be good style to
do so.

Regards,
Halil