Re: [RFC PATCH V2 0/7] Do not read from descripto ring

[Christoph Hellwig <hch@infradead.org>]

On Thu, May 06, 2021 at 04:12:17AM -0400, Michael S. Tsirkin wrote:
> Let's try for just a bit, won't make this window anyway:
> 
> I have an old idea. Add a way to find out that unmap is a nop
> (or more exactly does not use the address/length).
> Then in that case even with DMA API we do not need
> the extra data. Hmm?

So we actually do have a check for that from the early days of the DMA
API, but it only works at compile time: CONFIG_NEED_DMA_MAP_STATE.

But given how rare configs without an iommu or swiotlb are these days
it has stopped to be very useful.  Unfortunately a runtime-version is
not entirely trivial, but maybe if we allow for false positives we
could do something like this

bool dma_direct_need_state(struct device *dev)
{
	/* some areas could not be covered by any map at all */
	if (dev->dma_range_map)
		return false;
	if (force_dma_unencrypted(dev))
		return false;
	if (dma_direct_need_sync(dev))
		return false;
	return *dev->dma_mask == DMA_BIT_MASK(64);
}

bool dma_need_state(struct device *dev)
{
	const struct dma_map_ops *ops = get_dma_ops(dev);

	if (dma_map_direct(dev, ops))
		return dma_direct_need_state(dev);
	return ops->unmap_page ||
		ops->sync_single_for_cpu || ops->sync_single_for_device;
}