Re: [PATCH v6 4/7] vfio: ap: register IOMMU VFIO notifier

[Pierre Morel]

On 28/03/2019 21:46, Tony Krowiak wrote:
> On 3/22/19 10:43 AM, Pierre Morel wrote:
> > To be able to use the VFIO interface to facilitate the
> > mediated device memory pinning/unpinning we need to register
> > a notifier for IOMMU.
> >
> > While we will start to pin one guest page for the interrupt indicator
> > byte, this is still ok with ballooning as this page will never be
> > used by the guest virtio-balloon driver.
> > So the pinned page will never be freed. And even a broken guest does
> > so, that would not impact the host as the original page is still
> > in control by vfio.
>
> I apologize, but I do not understand what you are saying in the second
> sentence of the paragraph above. Why will the pinned page never be 
> freed?
Because it is in use by the guest's kernel as a notification information 
byte for the original PQAP AQIC.

 I understand that the pinned page is under the control of vfio
> until it is freed, but have no idea what you mean by "and even a broken
> guest does so"? A broken guest does what? Can you please reword this so
> it makes more sense?

A broken guest could free the page used for the NIB. What is obviously 
wrong.

> > Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
> > Reviewed-by: Cornelia Huck <cohuck@xxxxxxxxxx>
> > ---
> > Â drivers/s390/crypto/vfio_ap_ops.cÂÂÂÂ | 38 
> > +++++++++++++++++++++++++++++++++++
> > Â drivers/s390/crypto/vfio_ap_private.h |Â 2 ++
> > Â 2 files changed, 40 insertions(+)
> >
> > diff --git a/drivers/s390/crypto/vfio_ap_ops.c 
> > b/drivers/s390/crypto/vfio_ap_ops.c
> > index bdb36e0..3478499 100644
> > --- a/drivers/s390/crypto/vfio_ap_ops.c
> > +++ b/drivers/s390/crypto/vfio_ap_ops.c
> > @@ -787,6 +787,35 @@ static const struct attribute_group 
> > *vfio_ap_mdev_attr_groups[] = {
> > ÂÂÂÂÂ NULL
> > Â };
> > +/**
> > + * vfio_ap_mdev_iommu_notifier: IOMMU notifier callback
> > + *
> > + * @nb: The notifier block
> > + * @action: Action to be taken
> > + * @data: data associated with the request
> > + *
> > + * For an UNMAP request, unpin the guest IOVA (the NIB guest address we
> > + * pinned before). Other requests are ignored.
> > + *
> > + */
> > +static int vfio_ap_mdev_iommu_notifier(struct notifier_block *nb,
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unsigned long action, void *data)
> > +{
> > +ÂÂÂ struct ap_matrix_mdev *matrix_mdev;
> > +
> > +ÂÂÂ matrix_mdev = container_of(nb, struct ap_matrix_mdev, 
> > iommu_notifier);
> > +
>
> I don't understand why we registered this notifier. I may be wrong, but
> AFAIU, this notifier will be invoked only when the VFIO_IOMMU_UNMAP_DMA
> ioctl is called from userspace. I did an experiment and inserted some
> printf's to see if this ever gets called and verified it does not. Maybe
> you have a good reason of which I'm not aware. Can you enlighten me
> here?

The vfio_iommu_type1 pin page requires a notifier.

Regards,
Pierre

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