linux-kernel - Re: [PATCH 2/2] s390/vfio-ap: replace open coded locks for VFIO_GROUP_NOTIFY_SET

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Message-ID: <0f03ab0b-2dfd-e1c1-fe43-be2a59030a71@linux.ibm.com>
Date:   Thu, 22 Jul 2021 09:16:19 -0400
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     jjherne@...ux.ibm.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     borntraeger@...ibm.com, cohuck@...hat.com,
        pasic@...ux.vnet.ibm.com, jgg@...dia.com,
        alex.williamson@...hat.com, kwankhede@...dia.com, david@...hat.com
Subject: Re: [PATCH 2/2] s390/vfio-ap: replace open coded locks for
 VFIO_GROUP_NOTIFY_SET_KVM notification



On 7/21/21 3:37 PM, Jason J. Herne wrote:
> On 7/19/21 3:35 PM, Tony Krowiak wrote:
>> It was pointed out during an unrelated patch review that locks should 
>> not
>> be open coded - i.e., writing the algorithm of a standard lock in a
>> function instead of using a lock from the standard library. The 
>> setting and
>> testing of a busy flag and sleeping on a wait_event is the same thing
>> a lock does. The open coded locks are invisible to lockdep, so potential
>> locking problems are not detected.
>>
>> This patch removes the open coded locks used during
>> VFIO_GROUP_NOTIFY_SET_KVM notification. The busy flag
>> and wait queue were introduced to resolve a possible circular locking
>> dependency reported by lockdep when starting a secure execution guest
>> configured with AP adapters and domains. Reversing the order in which
>> the kvm->lock mutex and matrix_dev->lock mutex are locked resolves the
>> issue reported by lockdep, thus enabling the removal of the open coded
>> locks.
>>
>> Signed-off-by: Tony Krowiak <akrowiak@...ux.ibm.com>
>> ---
>>   arch/s390/kvm/kvm-s390.c              |  27 +++++-
>>   drivers/s390/crypto/vfio_ap_ops.c     | 132 ++++++++------------------
>>   drivers/s390/crypto/vfio_ap_private.h |   2 -
>>   3 files changed, 63 insertions(+), 98 deletions(-)
>>
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index a08f242a9f27..4d2ef3a3286e 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -2559,12 +2559,24 @@ static void kvm_s390_set_crycb_format(struct 
>> kvm *kvm)
>>           kvm->arch.crypto.crycbd |= CRYCB_FORMAT1;
>>   }
>>   +/*
>> + * kvm_arch_crypto_set_masks
>> + *
>> + * @kvm: a pointer to the object containing the crypto masks
>
> This should probably say "a pointer to the target guest's KVM struct" 
> or something to that effect. Same comment for the comment above 
> kvm_arch_crypto_clear_masks.

Will do.

>
>> + * @apm: the mask identifying the accessible AP adapters
>> + * @aqm: the mask identifying the accessible AP domains
>> + * @adm: the mask identifying the accessible AP control domains
>> + *
>> + * Set the masks that identify the adapters, domains and control 
>> domains to
>> + * which the KVM guest is granted access.
>> + *
>> + * Note: The kvm->lock mutex must be locked by the caller.
>> + */
>>   void kvm_arch_crypto_set_masks(struct kvm *kvm, unsigned long *apm,
>>                      unsigned long *aqm, unsigned long *adm)
>>   {
>>       struct kvm_s390_crypto_cb *crycb = kvm->arch.crypto.crycb;
>>   -    mutex_lock(&kvm->lock);
>>       kvm_s390_vcpu_block_all(kvm);
>>         switch (kvm->arch.crypto.crycbd & CRYCB_FORMAT_MASK) {
>> @@ -2595,13 +2607,21 @@ void kvm_arch_crypto_set_masks(struct kvm 
>> *kvm, unsigned long *apm,
>>       /* recreate the shadow crycb for each vcpu */
>>       kvm_s390_sync_request_broadcast(kvm, KVM_REQ_VSIE_RESTART);
>>       kvm_s390_vcpu_unblock_all(kvm);
>> -    mutex_unlock(&kvm->lock);
>>   }
>>   EXPORT_SYMBOL_GPL(kvm_arch_crypto_set_masks);
>>   +/*
>> + * kvm_arch_crypto_set_masks
>
> Copy/paste error here. Rename to kvm_arch_crypto_CLEAR_masks

I will fix it.

>
> I did not find anything else in my review. However, I'm still very new 
> to this code, so take that with a grain of salt :).

The grain of salt has been ingested.

>
> Also, I could not apply this to master. If there is a next version do 
> you mind rebasing? Seeing the patch in full context would be helpful.

This was rebased on the latest master branch at the time then re-tested. 
This is something I always
do prior to submitting patches, so is it possible you were using an 
older version of master?

>
>
>