Messages in this thread | | | Subject | Re: [PATCH v3] s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated | From | Tony Krowiak <> | Date | Wed, 16 Dec 2020 19:46:34 -0500 |
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On 12/16/20 11:05 AM, Christian Borntraeger 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@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 "(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. > > > 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 ?
I had that in the v2 patches, but mistakenly removed it because of a misinterpretation of the docs on posting a patch for a stable release. I'll restore it since I have to remove the unlock from the vfio_ap_mdev_unset_kvm function.
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