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

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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...)

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.
Once we merge domain_alloc() and finalize(), then this check disappears,
but we still need to test nr_endpoints in map/unmap to handle detached
domains (and we still need to fix the synchronization of nr_endpoints
against attach/detach). That's why I preferred doing this on viommu and
keeping it in one place.

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