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Wed, 16 Dec 2020 09:58:54 +0000 (GMT) Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 1441111C058; Wed, 16 Dec 2020 09:58:54 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.171.82.131]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 16 Dec 2020 09:58:54 +0000 (GMT) Subject: Re: [PATCH v3] s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated To: Halil Pasic Cc: Tony Krowiak , linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, stable@vger.kernel.org, gregkh@linuxfoundation.org, sashal@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> From: Christian Borntraeger Message-ID: Date: Wed, 16 Dec 2020 10:58:48 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 In-Reply-To: <20201216022140.02741788.pasic@linux.ibm.com> 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_04:2020-12-15,2020-12-16 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 adultscore=0 mlxscore=0 lowpriorityscore=0 spamscore=0 bulkscore=0 malwarescore=0 phishscore=0 mlxlogscore=999 suspectscore=0 priorityscore=1501 clxscore=1015 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2012160061 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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 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. > > 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 >