linux-kernel - Re: [PATCH v2 13/15] KVM: s390: Configure the guest's CRYCB

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Date:   Thu, 1 Mar 2018 10:37:46 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, kwankhede@...dia.com,
        bjsdjshi@...ux.vnet.ibm.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, pmorel@...ux.vnet.ibm.com,
        alifm@...ux.vnet.ibm.com, mjrosato@...ux.vnet.ibm.com,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com,
        pasic@...ux.vnet.ibm.com, fiuczy@...ux.vnet.ibm.com,
        buendgen@...ibm.com
Subject: Re: [PATCH v2 13/15] KVM: s390: Configure the guest's CRYCB

On 28.02.2018 21:45, Tony Krowiak wrote:
> On 02/28/2018 04:49 AM, David Hildenbrand wrote:
>>> +static int vfio_ap_mdev_open(struct mdev_device *mdev)
>>> +{
>>> +	struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
>>> +	unsigned long events;
>>> +	int ret;
>>> +
>>> +	matrix_mdev->group_notifier.notifier_call = vfio_ap_mdev_group_notifier;
>>> +	events = VFIO_GROUP_NOTIFY_SET_KVM;
>>> +	ret = vfio_register_notifier(mdev_dev(mdev), VFIO_GROUP_NOTIFY,
>>> +				     &events, &matrix_mdev->group_notifier);
>>> +
>>> +	ret = kvm_ap_configure_matrix(matrix_mdev->kvm,
>>> +				      matrix_mdev->matrix);
>>> +	if (ret)
>>> +		return ret;
>>> +
>>> +	ret = kvm_ap_enable_ie_mode(matrix_mdev->kvm);
>> Can't this happen while the guest is already running? Or what hinders us
>> from doing that?
> I'm not sure exactly what you're asking here. Are you asking if the 
> vfio_ap_mdev_open()
> function can be called multiple times while the guest is running? AFAIK 
> this will be
> called only once when the mediated device's file descriptor is opened. 
> This happens in
> QEMU when the -device vfio-ap device is realized.

Okay, but from a pure interface point of view, this could happen any
time, even while the guest is already running. Patching in the SCB of a
running VCPU is evil.

But I guess we don't have to worry about that when changing they way we
set ECA_APIE, as described in the other mail.

-- 

Thanks,

David / dhildenb