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Wed, 16 Dec 2020 16:05:25 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.171.82.131]) by d06av21.portsmouth.uk.ibm.com (Postfix) with ESMTP id 9EAFD52052; Wed, 16 Dec 2020 16:05:24 +0000 (GMT) Subject: Re: [PATCH v3] s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated From: Christian Borntraeger To: Halil Pasic Cc: Tony Krowiak , linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, cohuck@redhat.com, kwankhede@nvidia.com, pbonzini@redhat.com, alex.williamson@redhat.com, pasic@linux.vnet.ibm.com References: <20201214165617.28685-1-akrowiak@linux.ibm.com> <20201215115746.3552e873.pasic@linux.ibm.com> <44ffb312-964a-95c3-d691-38221cee2c0a@de.ibm.com> <20201216022140.02741788.pasic@linux.ibm.com> Message-ID: <1039a56a-f8d7-15f7-d6a6-cb126468bdff@de.ibm.com> Date: Wed, 16 Dec 2020 17:05:24 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-TM-AS-GCONF: 00 Content-Transfer-Encoding: 7bit X-Proofpoint-UnRewURL: 0 URL was un-rewritten MIME-Version: 1.0 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.343,18.0.737 definitions=2020-12-16_06:2020-12-15,2020-12-16 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 mlxscore=0 suspectscore=0 clxscore=1015 spamscore=0 priorityscore=1501 impostorscore=0 bulkscore=0 adultscore=0 phishscore=0 malwarescore=0 mlxlogscore=999 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2012160105 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 wrote: >> >>> >>> >>> On 15.12.20 11:57, Halil Pasic wrote: >>>> On Mon, 14 Dec 2020 11:56:17 -0500 >>>> Tony Krowiak 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 >>>>> --- >>>>> 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 ?