From: Yan Zhao <yan.y.zhao@intel.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: <kvm@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
<x86@kernel.org>, <alex.williamson@redhat.com>,
<kevin.tian@intel.com>, <iommu@lists.linux.dev>,
<pbonzini@redhat.com>, <seanjc@google.com>,
<dave.hansen@linux.intel.com>, <luto@kernel.org>,
<peterz@infradead.org>, <tglx@linutronix.de>, <mingo@redhat.com>,
<bp@alien8.de>, <hpa@zytor.com>, <corbet@lwn.net>,
<joro@8bytes.org>, <will@kernel.org>, <robin.murphy@arm.com>,
<baolu.lu@linux.intel.com>, <yi.l.liu@intel.com>
Subject: Re: [PATCH 5/5] iommufd: Flush CPU caches on DMA pages in non-coherent domains
Date: Mon, 13 May 2024 15:43:45 +0800 [thread overview]
Message-ID: <ZkHEsfaGAXuOFMkq@yzhao56-desk.sh.intel.com> (raw)
In-Reply-To: <20240510132928.GS4650@nvidia.com>
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.
next prev parent reply other threads:[~2024-05-13 7:44 UTC|newest]
Thread overview: 78+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-07 6:18 [PATCH 0/5] Enforce CPU cache flush for non-coherent device assignment Yan Zhao
2024-05-07 6:19 ` [PATCH 1/5] x86/pat: Let pat_pfn_immune_to_uc_mtrr() check MTRR for untracked PAT range Yan Zhao
2024-05-07 8:26 ` Tian, Kevin
2024-05-07 9:12 ` Yan Zhao
2024-05-08 22:14 ` Alex Williamson
2024-05-09 3:36 ` Yan Zhao
2024-05-16 7:42 ` Tian, Kevin
2024-05-16 14:07 ` Sean Christopherson
2024-05-20 2:36 ` Tian, Kevin
2024-05-07 6:20 ` [PATCH 2/5] KVM: x86/mmu: Fine-grained check of whether a invalid & RAM PFN is MMIO Yan Zhao
2024-05-07 8:39 ` Tian, Kevin
2024-05-07 9:19 ` Yan Zhao
2024-05-07 6:20 ` [PATCH 3/5] x86/mm: Introduce and export interface arch_clean_nonsnoop_dma() Yan Zhao
2024-05-07 8:51 ` Tian, Kevin
2024-05-07 9:40 ` Yan Zhao
2024-05-20 14:07 ` Christoph Hellwig
2024-05-21 15:49 ` Jason Gunthorpe
2024-05-21 16:00 ` Jason Gunthorpe
2024-05-22 3:41 ` Yan Zhao
2024-05-28 6:37 ` Christoph Hellwig
2024-06-01 19:46 ` Jason Gunthorpe
2024-06-06 2:48 ` Yan Zhao
2024-06-06 11:55 ` Jason Gunthorpe
2024-06-07 9:39 ` Yan Zhao
2024-05-07 6:21 ` [PATCH 4/5] vfio/type1: Flush CPU caches on DMA pages in non-coherent domains Yan Zhao
2024-05-09 18:10 ` Alex Williamson
2024-05-10 10:31 ` Yan Zhao
2024-05-10 16:57 ` Alex Williamson
2024-05-13 7:11 ` Yan Zhao
2024-05-16 7:53 ` Tian, Kevin
2024-05-16 8:34 ` Tian, Kevin
2024-05-16 20:31 ` Alex Williamson
2024-05-17 17:11 ` Jason Gunthorpe
2024-05-20 2:52 ` Tian, Kevin
2024-05-21 16:07 ` Jason Gunthorpe
2024-05-21 16:21 ` Alex Williamson
2024-05-21 16:34 ` Jason Gunthorpe
2024-05-21 18:19 ` Alex Williamson
2024-05-21 18:37 ` Jason Gunthorpe
2024-05-22 6:24 ` Tian, Kevin
2024-05-22 12:29 ` Jason Gunthorpe
2024-05-22 14:43 ` Alex Williamson
2024-05-22 16:52 ` Jason Gunthorpe
2024-05-22 18:22 ` Alex Williamson
2024-05-22 23:26 ` Tian, Kevin
2024-05-22 23:32 ` Jason Gunthorpe
2024-05-22 23:40 ` Tian, Kevin
2024-05-23 14:58 ` Jason Gunthorpe
2024-05-23 22:47 ` Alex Williamson
2024-05-24 0:30 ` Tian, Kevin
2024-05-24 13:50 ` Jason Gunthorpe
2024-05-22 3:33 ` Yan Zhao
2024-05-22 3:24 ` Yan Zhao
2024-05-22 12:26 ` Jason Gunthorpe
2024-05-24 3:07 ` Yan Zhao
2024-05-16 20:50 ` Alex Williamson
2024-05-17 3:11 ` Yan Zhao
2024-05-17 4:44 ` Alex Williamson
2024-05-17 5:00 ` Yan Zhao
2024-05-07 6:22 ` [PATCH 5/5] iommufd: " Yan Zhao
2024-05-09 14:13 ` Jason Gunthorpe
2024-05-10 8:03 ` Yan Zhao
2024-05-10 13:29 ` Jason Gunthorpe
2024-05-13 7:43 ` Yan Zhao [this message]
2024-05-14 15:11 ` Jason Gunthorpe
2024-05-15 7:06 ` Yan Zhao
2024-05-15 20:43 ` Jason Gunthorpe
2024-05-16 2:32 ` Yan Zhao
2024-05-16 8:38 ` Tian, Kevin
2024-05-16 9:48 ` Yan Zhao
2024-05-17 17:04 ` Jason Gunthorpe
2024-05-20 2:45 ` Yan Zhao
2024-05-21 16:04 ` Jason Gunthorpe
2024-05-22 3:17 ` Yan Zhao
2024-05-22 6:29 ` Yan Zhao
2024-05-22 17:01 ` Jason Gunthorpe
2024-05-27 7:15 ` Yan Zhao
2024-06-01 16:48 ` Jason Gunthorpe
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