Re: [PATCH 1/2] iommu: Prevent RESV_DIRECT devices from blocking domains

[Jason Gunthorpe]

On Mon, Jun 19, 2023 at 02:33:18PM +0100, Robin Murphy wrote:
> > @@ -2121,6 +2125,21 @@ static int __iommu_device_set_domain(struct iommu_group *group,
> >   {
> >   	int ret;
> > +	/*
> > +	 * If the driver has requested IOMMU_RESV_DIRECT then we cannot allow
> > +	 * the blocking domain to be attached as it does not contain the
> > +	 * required 1:1 mapping. This test effectively exclusive the device from
> > +	 * being used with iommu_group_claim_dma_owner() which will block vfio
> > +	 * and iommufd as well.
> > +	 */
> > +	if (dev->iommu->requires_direct &&
> > +	    (new_domain->type == IOMMU_DOMAIN_BLOCKED ||
> 
> Given the notion elsewhere that we want to use the blocking domain as a last
> resort to handle an attach failure,

We shouldn't do that for cases where requires_direct is true, the last
resort will have to be the static identity domain.

> at face value it looks suspect that failing to attach to a blocking
> domain could also be a thing. I guess technically this is failing at
> a slightly different level so maybe it does work out OK, but it's
> still smelly.

It basically says that this driver doesn't support blocking domains on
this device. What we don't want is for the driver to fail blocking or
identity attaches.
 
> The main thing, though, is that not everything implements the
> IOMMU_DOMAIN_BLOCKED optimisation, so a nominal blocking domain could be
> IOMMU_DOMAIN_UNMANAGED as well. 

Yes, it should check new_domain == group->blocking_domain as well.

> FWIW I'd prefer to make the RESV_DIRECT check explicit in
> __iommu_take_dma_ownership() rather than hide it in an
> implementation detail; that's going to be a lot clearer to reason
> about as time goes on.

We want to completely forbid blocking domains at all on these devices
because they are not supported (by FW request). I don't really like
the idea that we go and assume the only users of blocking domains are
also using take_dma_ownership() - that feels like a future bug waiting
to happen.

Jason