linux-kernel - Re: [PATCH v5 4/7] s390: ap: setup relation betwen KVM and mediated device

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Date:   Tue, 19 Mar 2019 17:48:24 +0100
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     borntraeger@...ibm.com, alex.williamson@...hat.com,
        cohuck@...hat.com, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        frankja@...ux.ibm.com, akrowiak@...ux.ibm.com, david@...hat.com,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        freude@...ux.ibm.com, mimu@...ux.ibm.com
Subject: Re: [PATCH v5 4/7] s390: ap: setup relation betwen KVM and mediated
 device

On 19/03/2019 16:27, Halil Pasic wrote:
> On Tue, 19 Mar 2019 15:47:05 +0100
> Pierre Morel <pmorel@...ux.ibm.com> wrote:
> 
>>>>>>>         if (matrix_mdev->kvm)
>>>>>>>             kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
>>>>>>
>>>>>> This still conditional?
>>>>>
>>>>> Yes, nothing to clear if there is no KVM.
>>>>>   
>>>>
>>>> Since we have ensured the open only works if there is a KVM at that
>>>> point in time, and we have taken a reference to KVM, I would expect
>>>> KVM can not go away before we give up our reference.
>>>
>>> Right.
>>
>> Right but based on the assumption we do a kvm_get_kvm() during open.
>>
>> But now we will do it inside the notifier, so the logic is to do a
>> kvm_put_kvm in the notifier too.
>> This is important because userland will ask us to release the KVM/VFIO
>> link through this notifier.
>> So I will have to rework this part where KVM==NULL in the notifier too.
>>
>> Regards,
>> Pierre
> 
> I think it can be done both ways. If you ensure KVM != NULL if the open
> succeeds and take the reference in the notifier. I suppose if open()
> fails release() won't be called. But the logic/code in open() would get
> quite ugly because the callback could be called assync so that it
> overlaps with the rest of open().

Not necessary, but there is more than just the kvm_get_kvm().

When the user calls KVM_DEV_VFIO_GROUP_DEL he asks to break the link 
between VFIO and KVM.

Currently we just ignore this instead of stopping all activity 
associated with KVM.

But we have more bugs there:
We should not support multiple open of the mdev which will overwrite 
matrix->kvm for the same mdev with a different KVM.
I send a bugfix for this.


> 
> Not failing open() in case of no KVM is there yet is in my opinion
> cleaner anyway.

If we handle correctly the notifiers and the exclusivity, we can do this.
I will make this correctly for the next iteration.

Regards,
Pierre

> 
> Regards,
> Halil
> 


-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany