Re: [PATCH 07/18] dma-mapping: Implement link/unlink ranges API

[Leon Romanovsky <leon@kernel.org>]

On Mon, Oct 28, 2024 at 10:00:25AM +0800, Baolu Lu wrote:
> On 2024/10/27 22:21, Leon Romanovsky wrote:
> > +/**
> > + * dma_iova_sync - Sync IOTLB
> > + * @dev: DMA device
> > + * @state: IOVA state
> > + * @offset: offset into the IOVA state to sync
> > + * @size: size of the buffer
> > + * @ret: return value from the last IOVA operation
> > + *
> > + * Sync IOTLB for the given IOVA state. This function should be called on
> > + * the IOVA-contigous range created by one ore more dma_iova_link() calls
> > + * to sync the IOTLB.
> > + */
> > +int dma_iova_sync(struct device *dev, struct dma_iova_state *state,
> > +		size_t offset, size_t size, int ret)
> > +{
> > +	struct iommu_domain *domain = iommu_get_dma_domain(dev);
> > +	struct iommu_dma_cookie *cookie = domain->iova_cookie;
> > +	struct iova_domain *iovad = &cookie->iovad;
> > +	dma_addr_t addr = state->addr + offset;
> > +	size_t iova_start_pad = iova_offset(iovad, addr);
> > +
> > +	addr -= iova_start_pad;
> > +	size = iova_align(iovad, size + iova_start_pad);
> > +
> > +	if (!ret)
> > +		ret = iommu_sync_map(domain, addr, size);
> > +	if (ret)
> > +		iommu_unmap(domain, addr, size);
> 
> It appears strange that mapping is not done in this helper, but
> unmapping is added in the failure path. Perhaps I overlooked anything?

Like iommu_sync_map() is performed on whole continuous range, the iommu_unmap()
should be done on the same range. So, technically you can unmap only part of
the range which called to dma_iova_link() and failed, but you will need
to make sure that iommu_sync_map() is still called for "successful" part of
iommu_map().

In that case, you will need to undo everything anyway and it means that
you will call to iommu_unmap() on the successful part of the range
anyway.

dma_iova_sync() is single operation for the whole range and
iommu_unmap() too, so they are bound together.

> To my understanding, it should like below:
> 
> 	return iommu_sync_map(domain, addr, size);
> 
> In the drivers that make use of this interface should do something like
> below:
> 
> 	ret = dma_iova_sync(...);
> 	if (ret)
> 		dma_iova_destroy(...)

It is actually what is happening in the code, but in less direct way due
to unwinding of the code.

As an simple example, see VFIO patch https://lore.kernel.org/all/0a517ddff099c14fac1ceb0e75f2f50ed183d09c.1730037276.git.leon@kernel.org/
where failed in dma_iova_sync() will trigger call to unregister_dma_pages() and that will call to dma_iova_destroy().

> 
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(dma_iova_sync);
> 
> Thanks,
> baolu
>