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

Yan Zhao
2024-05-13T07:44:34Z

On Fri, May 10, 2024 at 10:29:28AM -0300, Jason Gunthorpe wrote:
> On Fri, May 10, 2024 at 04:03:04PM +0800, Yan Zhao wrote:
> > > > @@ -1358,10 +1377,17 @@ int iopt_area_fill_domain(struct iopt_area *area, struct iommu_domain *domain)
> > > > {
> > > > unsigned long done_end_index;
> > > > struct pfn_reader pfns;
> > > > + bool cache_flush_required;
> > > > int rc;
> > > >
> > > > lockdep_assert_held(&area->pages->mutex);
> > > >
> > > > + cache_flush_required = area->iopt->noncoherent_domain_cnt &&
> > > > + !area->pages->cache_flush_required;
> > > > +
> > > > + if (cache_flush_required)
> > > > + area->pages->cache_flush_required = true;
> > > > +
> > > > rc = pfn_reader_first(&pfns, area->pages, iopt_area_index(area),
> > > > iopt_area_last_index(area));
> > > > if (rc)
> > > > @@ -1369,6 +1395,9 @@ int iopt_area_fill_domain(struct iopt_area *area, struct iommu_domain *domain)
> > > >
> > > > while (!pfn_reader_done(&pfns)) {
> > > > done_end_index = pfns.batch_start_index;
> > > > + if (cache_flush_required)
> > > > + iopt_cache_flush_pfn_batch(&pfns.batch);
> > > > +
> > >
> > > This is a bit unfortunate, it means we are going to flush for every
> > > domain, even though it is not required. I don't see any easy way out
> > > of that :(
> > Yes. Do you think it's possible to add an op get_cache_coherency_enforced
> > to iommu_domain_ops?
>
> Do we need that? The hwpt already keeps track of that? the enforced could be
> copied into the area along side storage_domain
>
> Then I guess you could avoid flushing in the case the page came from
> the storage_domain...
>
> You'd want the storage_domain to preferentially point to any
> non-enforced domain.
>
> Is it worth it? How slow is this stuff?
Sorry, I might have misunderstood your intentions in my previous mail.
In iopt_area_fill_domain(), flushing CPU caches is only performed when
(1) noncoherent_domain_cnt is non-zero and
(2) area->pages->cache_flush_required is false.
area->pages->cache_flush_required is also set to true after the two are met, so
that the next flush to the same "area->pages" in filling phase will be skipped.

In my last mail, I thought you wanted to flush for every domain even if
area->pages->cache_flush_required is true, because I thought that you were
worried about that checking area->pages->cache_flush_required might results in
some pages, which ought be flushed, not being flushed.
So, I was wondering if we could do the flush for every non-coherent domain by
checking whether domain enforces cache coherency.

However, as you said, we can check hwpt instead if it's passed in
iopt_area_fill_domain().

On the other side, after a second thought, looks it's still good to check
area->pages->cache_flush_required?
- "area" and "pages" are 1:1. In other words, there's no such a condition that
several "area"s are pointing to the same "pages".
Is this assumption right?
- Once area->pages->cache_flush_required is set to true, it means all pages
indicated by "area->pages" has been mapped into a non-coherent domain
(though the domain is not necessarily the storage domain).
Is this assumption correct as well?
If so, we can safely skip the flush in iopt_area_fill_domain() if
area->pages->cache_flush_required is true.