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

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Message-ID: <56f576ba-4eee-696e-45aa-f4db1e96bd57@redhat.com>
Date:   Fri, 2 Mar 2018 11:08:12 +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 01.03.2018 21:42, Tony Krowiak wrote:
> 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?

It can't happen right now (the way QEMU uses it), but the kernel
interface allows it, no?

Anyhow, as discussed this should be handled directly while creating a
VCPU. Then also CPU hotplug is properly covered.


-- 

Thanks,

David / dhildenb