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Subject: Re: [PATCH v11 07/14] s390/vfio-ap: sysfs attribute to display the
 guest's matrix
To:     Halil Pasic <pasic@linux.ibm.com>
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, freude@linux.ibm.com, borntraeger@de.ibm.com,
        cohuck@redhat.com, mjrosato@linux.ibm.com,
        alex.williamson@redhat.com, kwankhede@nvidia.com,
        fiuczy@linux.ibm.com, frankja@linux.ibm.com, david@redhat.com,
        hca@linux.ibm.com, gor@linux.ibm.com
References: <20201022171209.19494-1-akrowiak@linux.ibm.com>
 <20201022171209.19494-8-akrowiak@linux.ibm.com>
 <20201028091758.73aa77a3.pasic@linux.ibm.com>
 <b96fe876-c67a-fe6c-0e3a-7b4948edeef4@linux.ibm.com>
 <20201114001248.3b397c8c.pasic@linux.ibm.com>
From:   Tony Krowiak <akrowiak@linux.ibm.com>
Message-ID: <d25a6810-00df-a2fc-4541-548917fdcc40@linux.ibm.com>
Date:   Thu, 19 Nov 2020 13:15:25 -0500
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On 11/13/20 6:12 PM, Halil Pasic wrote:
> On Fri, 13 Nov 2020 12:27:32 -0500
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>>
>> On 10/28/20 4:17 AM, Halil Pasic wrote:
>>> On Thu, 22 Oct 2020 13:12:02 -0400
>>> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>>>
>>>> +static ssize_t guest_matrix_show(struct device *dev,
>>>> +				 struct device_attribute *attr, char *buf)
>>>> +{
>>>> +	ssize_t nchars;
>>>> +	struct mdev_device *mdev = mdev_from_dev(dev);
>>>> +	struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
>>>> +
>>>> +	if (!vfio_ap_mdev_has_crycb(matrix_mdev))
>>>> +		return -ENODEV;
>>> I'm wondering, would it make sense to have guest_matrix display the would
>>> be guest matrix when we don't have a KVM? With the filtering in
>>> place, the question in what guest_matrix would my (assign) matrix result
>>> right now if I were to hook up my vfio_ap_mdev to a guest seems a
>>> legitimate one.
>> A couple of thoughts here:
>> * The ENODEV informs the user that there is no guest running
>>      which makes sense to me given this interface displays the
>>      guest matrix. The alternative, which I considered, was to
>>      display an empty matrix (i.e., nothing).
>> * This would be a pretty drastic change to the design because
>>      the shadow_apcb - which is what is displayed via this interface - is
>>      only updated when the guest is started and while it is running (i.e.,
>>      hot plug of new adapters/domains). Making this change would
>>      require changing that entire design concept which I am reluctant
>>      to do at this point in the game.
>>
>>
> No problem. My thinking was, that, because we can do the
> assign/unassing ops also for the running guest, that we also have
> the code to do the maintenance on the shadow_apcb. In this
> series this code is conditional with respect to vfio_ap_mdev_has_crycb().
> E.g.
>
> static ssize_t assign_adapter_store(struct device *dev,
>                                      struct device_attribute *attr,
>                                      const char *buf, size_t count)
> {
> [..]
>          if (vfio_ap_mdev_has_crycb(matrix_mdev))
>                  if (vfio_ap_mdev_filter_guest_matrix(matrix_mdev, true))
>                          vfio_ap_mdev_commit_shadow_apcb(matrix_mdev);
>
> If one were to move the
> vfio_ap_mdev_has_crycb() check into vfio_ap_mdev_commit_shadow_apcb()
> then we would have an always up to date shatdow_apcb, we could display.
>
> I don't feel strongly about this. Was just an idea, because if the result
> of the filtering is surprising, currently the only to see, without
> knowing the algorithm, and possibly the state, and the history of the
> system, is to actually start a guest.

Okay, I can buy this and will make the change.

>
> Regards,
> Halil
>