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Subject: Re: [PATCH v2 13/15] KVM: s390: Configure the guest's CRYCB
To:     David Hildenbrand <david@redhat.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
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From:   Tony Krowiak <akrowiak@linux.vnet.ibm.com>
Date:   Thu, 1 Mar 2018 15:42:32 -0500
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On 03/01/2018 04:37 AM, David Hildenbrand wrote:
> 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.
How can this happen while the guest is running? QEMU opens the fd when the
device is realized and AFAIK vfio mdev will not allow any other process to
open it until the guest is terminated. What am I missing?
>
> 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.
>