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

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Date: Mon, 13 May 2024 15:43:45 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
	<alex.williamson@...hat.com>, <kevin.tian@...el.com>,
	<iommu@...ts.linux.dev>, <pbonzini@...hat.com>, <seanjc@...gle.com>,
	<dave.hansen@...ux.intel.com>, <luto@...nel.org>, <peterz@...radead.org>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>, <hpa@...or.com>,
	<corbet@....net>, <joro@...tes.org>, <will@...nel.org>,
	<robin.murphy@....com>, <baolu.lu@...ux.intel.com>, <yi.l.liu@...el.com>
Subject: Re: [PATCH 5/5] iommufd: Flush CPU caches on DMA pages in
 non-coherent domains

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.