On Thu, May 06, 2021 at 01:38:29PM +0100, Christoph Hellwig wrote:
> 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;
> }
Yea that sounds like a good idea. We will need to document that.
Something like:
/*
* dma_need_state - report whether unmap calls use the address and length
* @dev: device to guery
*
* This is a runtime version of CONFIG_NEED_DMA_MAP_STATE.
*
* Return the value indicating whether dma_unmap_* and dma_sync_* calls for the device
* use the DMA state parameters passed to them.
* The DMA state parameters are: scatter/gather list/table, address and
* length.
*
* If dma_need_state returns false then DMA state parameters are
* ignored by all dma_unmap_* and dma_sync_* calls, so it is safe to pass 0 for
* address and length, and DMA_UNMAP_SG_TABLE_INVALID and
* DMA_UNMAP_SG_LIST_INVALID for s/g table and length respectively.
* If dma_need_state returns true then DMA state might
* be used and so the actual values are required.
*/
And we will need DMA_UNMAP_SG_TABLE_INVALID and
DMA_UNMAP_SG_LIST_INVALID as pointers to an empty global table and list
for calls such as dma_unmap_sgtable that dereference pointers before checking
they are used.
Does this look good?
The table/length variants are for consistency, virtio specifically does
not use s/g at the moment, but it seems nicer than leaving
users wonder what to do about these.
Thoughts? Jason want to try implementing?
--
MST
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