Re: [PATCH] dma: DMA_ATTR_SKIP_CPU_SYNC documentation tweaks

[Christoph Hellwig]

On Thu, Jul 20, 2023 at 02:21:05AM -0400, Michael S. Tsirkin wrote:
> On Thu, Jul 20, 2023 at 08:07:42AM +0200, Christoph Hellwig wrote:
> > On Wed, Jul 19, 2023 at 06:15:59AM -0400, Michael S. Tsirkin wrote:
> > > A recent patchset highlighted to me that DMA_ATTR_SKIP_CPU_SYNC
> > > might be easily misunderstood.
> > 
> > .. just curious: what patchset is that?  DMA_ATTR_SKIP_CPU_SYNC is
> > often a bad idea and all users probably could use a really good
> > audit..
> 
> Message-Id: <20230710034237.12391-1-xuanzhuo@xxxxxxxxxxxxxxxxx>

Do you have an actual link?

> 
> 
> Looks like there's really little else can be done: there's a
> shared page we allow DMA into, so we sync periodically.
> Then when we unmap we really do not need that data
> synced again.
> 
> What exactly is wrong with this?

A "shared" page without ownership can't work with the streaming
DMA API (dma_map_*) at all.  You need to use dma_alloc_coherent
so that it is mapped uncached.