Virtualization - Sep 2023 - [PATCH v2 1/2] iommu/virtio: Make use of ops->iotlb_sync_map

On Tue, Sep 19, 2023 at 11:46:49AM -0300, Jason Gunthorpe
wrote:
> On Tue, Sep 19, 2023 at 09:15:19AM +0100, Jean-Philippe Brucker wrote:
> > On Mon, Sep 18, 2023 at 05:37:47PM +0100, Robin Murphy wrote:
> > > > diff --git a/drivers/iommu/virtio-iommu.c
b/drivers/iommu/virtio-iommu.c
> > > > index 17dcd826f5c2..3649586f0e5c 100644
> > > > --- a/drivers/iommu/virtio-iommu.c
> > > > +++ b/drivers/iommu/virtio-iommu.c
> > > > @@ -189,6 +189,12 @@ static int viommu_sync_req(struct
viommu_dev *viommu)
> > > >   	int ret;
> > > >   	unsigned long flags;
> > > > +	/*
> > > > +	 * .iotlb_sync_map and .flush_iotlb_all may be called
before the viommu
> > > > +	 * is initialized e.g. via
iommu_create_device_direct_mappings()
> > > > +	 */
> > > > +	if (!viommu)
> > > > +		return 0;
> > > 
> > > Minor nit: I'd be inclined to make that check explicitly in
the places where
> > > it definitely is expected, rather than allowing *any* sync to
silently do
> > > nothing if called incorrectly. Plus then they could use
> > > vdomain->nr_endpoints for consistency with the equivalent
checks elsewhere
> > > (it did take me a moment to figure out how we could get to
.iotlb_sync_map
> > > with a NULL viommu without viommu_map_pages() blowing up
first...)
> 
> This makes more sense to me
> 
> Ultimately this driver should reach a point where every iommu_domain
> always has a non-null domain->viommu because it will be set during
> alloc.
> 
> But it can still have nr_endpoints == 0, doesn't it make sense to
> avoid sync in this case?
> 
> (btw this driver is missing locking around vdomain->nr_endpoints)
Yes, that's on my list
> 
> > They're not strictly equivalent: this check works around a
temporary issue
> > with the IOMMU core, which calls map/unmap before the domain is
> > finalized.
> 
> Where? The above points to iommu_create_device_direct_mappings() but
> it doesn't because the pgsize_bitmap == 0:
__iommu_domain_alloc() sets pgsize_bitmap in this case:

        /*
         * If not already set, assume all sizes by default; the driver
         * may override this later
         */
        if (!domain->pgsize_bitmap)
                domain->pgsize_bitmap = bus->iommu_ops->pgsize_bitmap;

Thanks,
Jean

On Fri, Sep 22, 2023 at 08:57:19AM +0100, Jean-Philippe Brucker
wrote:
> > > They're not strictly equivalent: this check works around a
temporary issue
> > > with the IOMMU core, which calls map/unmap before the domain is
> > > finalized.
> > 
> > Where? The above points to iommu_create_device_direct_mappings() but
> > it doesn't because the pgsize_bitmap == 0:
> 
> __iommu_domain_alloc() sets pgsize_bitmap in this case:
> 
>         /*
>          * If not already set, assume all sizes by default; the driver
>          * may override this later
>          */
>         if (!domain->pgsize_bitmap)
>                 domain->pgsize_bitmap =
bus->iommu_ops->pgsize_bitmap;
Dirver's shouldn't do that.

The core code was fixed to try again with mapping reserved regions to
support these kinds of drivers.

Jason