Re: [PATCH 5/5] iommufd: Flush CPU caches on DMA pages in non-coherent domains

Yan Zhao
2024-05-16T02:33:48Z

On Wed, May 15, 2024 at 05:43:04PM -0300, Jason Gunthorpe wrote:
> On Wed, May 15, 2024 at 03:06:36PM +0800, Yan Zhao wrote:
>
> > > So it has to be calculated on closer to a page by page basis (really a
> > > span by span basis) if flushing of that span is needed based on where
> > > the pages came from. Only pages that came from a hwpt that is
> > > non-coherent can skip the flushing.
> > Is area by area basis also good?
> > Isn't an area either not mapped to any domain or mapped into all domains?
>
> Yes, this is what the span iterator turns into in the background, it
> goes area by area to cover things.
>
> > But, yes, considering the limited number of non-coherent domains, it appears
> > more robust and clean to always flush for non-coherent domain in
> > iopt_area_fill_domain().
> > It eliminates the need to decide whether to retain the area flag during a split.
>
> And flush for pin user pages, so you basically always flush because
> you can't tell where the pages came from.
As a summary, do you think it's good to flush in below way?

1. in iopt_area_fill_domains(), flush before mapping a page into domains when
iopt->noncoherent_domain_cnt > 0, no matter where the page is from.
Record cache_flush_required in pages for unpin.
2. in iopt_area_fill_domain(), pass in hwpt to check domain non-coherency.
flush before mapping a page into a non-coherent domain, no matter where the
page is from.
Record cache_flush_required in pages for unpin.
3. in batch_unpin(), flush if pages->cache_flush_required before
unpin_user_pages.