Re: [PATCH] s390: protvirt: virtio: Refuse device without IOMMU

[Pierre Morel]

On 2020-06-11 05:10, Jason Wang wrote:
> On 2020/6/10 äå9:11, Pierre Morel wrote:
> > Protected Virtualisation protects the memory of the guest and
> > do not allow a the host to access all of its memory.
> >
> > Let's refuse a VIRTIO device which does not use IOMMU
> > protected access.
> >
> > Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
> > ---
> > Â drivers/s390/virtio/virtio_ccw.c | 5 +++++
> > Â 1 file changed, 5 insertions(+)
> >
> > diff --git a/drivers/s390/virtio/virtio_ccw.c 
> > b/drivers/s390/virtio/virtio_ccw.c
> > index 5730572b52cd..06ffbc96587a 100644
> > --- a/drivers/s390/virtio/virtio_ccw.c
> > +++ b/drivers/s390/virtio/virtio_ccw.c
> > @@ -986,6 +986,11 @@ static void virtio_ccw_set_status(struct 
> > virtio_device *vdev, u8 status)
> > ÂÂÂÂÂ if (!ccw)
> > ÂÂÂÂÂÂÂÂÂ return;
> > +ÂÂÂ /* Protected Virtualisation guest needs IOMMU */
> > +ÂÂÂ if (is_prot_virt_guest() &&
> > +ÂÂÂÂÂÂÂ !__virtio_test_bit(vdev, VIRTIO_F_IOMMU_PLATFORM))
> > +ÂÂÂÂÂÂÂÂÂÂÂ status &= ~VIRTIO_CONFIG_S_FEATURES_OK;
> > +
> > ÂÂÂÂÂ /* Write the status to the host. */
> > ÂÂÂÂÂ vcdev->dma_area->status = status;
> > ÂÂÂÂÂ ccw->cmd_code = CCW_CMD_WRITE_STATUS;
>
>
> I wonder whether we need move it to virtio core instead of ccw.
>
> I think the other memory protection technologies may suffer from this as 
> well.
>
> Thanks


What would you think of the following, also taking into account Connie's 
comment on where the test should be done:

- declare a weak function in virtio.c code, returning that memory 
protection is not in use.

- overwrite the function in the arch code

- call this function inside core virtio_finalize_features() and if 
required fail if the device don't have VIRTIO_F_IOMMU_PLATFORM.

Alternative could be to test a global variable that the architecture 
would overwrite if needed but I find the weak function solution more 
flexible.

With a function, we also have the possibility to provide the device as 
argument and take actions depending it, this may answer Halil's concern.

Regards,
Pierre

--
Pierre Morel
IBM Lab Boeblingen