linux-kernel - RE: [PATCH 4/5] vfio/type1: Flush CPU caches on DMA pages in non-coherent domains

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Date: Thu, 16 May 2024 07:53:03 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: "Zhao, Yan Y" <yan.y.zhao@...el.com>, Alex Williamson
	<alex.williamson@...hat.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>, "jgg@...dia.com" <jgg@...dia.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"luto@...nel.org" <luto@...nel.org>, "peterz@...radead.org"
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	"robin.murphy@....com" <robin.murphy@....com>, "baolu.lu@...ux.intel.com"
	<baolu.lu@...ux.intel.com>, "Liu, Yi L" <yi.l.liu@...el.com>
Subject: RE: [PATCH 4/5] vfio/type1: Flush CPU caches on DMA pages in
 non-coherent domains

> From: Zhao, Yan Y <yan.y.zhao@...el.com>
> Sent: Monday, May 13, 2024 3:11 PM
> On Fri, May 10, 2024 at 10:57:28AM -0600, Alex Williamson wrote:
> > On Fri, 10 May 2024 18:31:13 +0800
> > Yan Zhao <yan.y.zhao@...el.com> wrote:
> > >
> > > The dma->cache_flush_required is to mark whether pages in a vfio_dma
> requires
> > > cache flush in the subsequence mapping into the first non-coherent
> domain
> > > and page unpinning.
> >
> > How do we arrive at a sequence where we have dma-
> >cache_flush_required
> > that isn't the result of being mapped into a domain with
> > !domain->enforce_cache_coherency?
> Hmm, dma->cache_flush_required IS the result of being mapped into a
> domain with
> !domain->enforce_cache_coherency.
> My concern only arrives from the actual code sequence, i.e.
> dma->cache_flush_required is set to true before the actual mapping.
> 
> If we rename it to dma->mapped_noncoherent and only set it to true after
> the
> actual successful mapping, it would lead to more code to handle flushing for
> the
> unwind case.
> Currently, flush for unwind is handled centrally in vfio_unpin_pages_remote()
> by checking dma->cache_flush_required, which is true even before a full
> successful mapping, so we won't miss flush on any pages that are mapped
> into a
> non-coherent domain in a short window.
> 

What about storing a vfio_iommu pointer in vfio_dma? Or pass an extra
parameter to vfio_unpin_pages_remote()...