dma_mmap_fault discussion

From: Thomas HellstrÃm (VMware)
Date: Thu Sep 05 2019 - 13:05:56 EST

Hi, Christoph,

On 9/5/19 5:24 PM, Christoph Hellwig wrote:
On Thu, Sep 05, 2019 at 05:21:24PM +0200, Thomas HellstrÃm (VMware) wrote:
On 9/5/19 4:15 PM, Dave Hansen wrote:
Hi Thomas,

Thanks for the second batch of patches! These look much improved on all
Yes, although the TTM functionality isn't in yet. Hopefully we won't have to
bother you with those though, since this assumes TTM will be using the dma
Please take a look at dma_mmap_prepare and dma_mmap_fault in this

they should allow to fault dma api pages in the page fault handler. But
this is totally hot off the press and not actually tested for the last
few patches. Note that I've also included your two patches from this
series to handle SEV.

I took a quick look at the interfaces and have two questions:

1) dma_mmap_prepare(), would it be possible to drop the references to the actual coherent region? The thing is that TTM doesn't know at mmap time what memory will be backing the region. It can be VRAM, coherent memory or system memory. Also see below:

2) @cpu_addr and @dma_addr are the values pointing at the beginning of an allocated chunk, right? The reason I'm asking is that TTM's coherent memory pool is sub-allocating from larger chunks into smaller PAGE_SIZE chunks, which means that a TTM buffer object may be randomly split across larger chunks, which means we have to store these values for each PAGE_SiZE chunk.