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Date: Tue, 21 May 2024 13:00:16 -0300
From: Jason Gunthorpe <jgg@nvidia.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Yan Zhao <yan.y.zhao@intel.com>, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	alex.williamson@redhat.com, kevin.tian@intel.com,
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	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,
	Russell King <linux@armlinux.org.uk>
Subject: Re: [PATCH 3/5] x86/mm: Introduce and export interface
 arch_clean_nonsnoop_dma()
Message-ID: <20240521160016.GA2513156@nvidia.com>
References: <20240507061802.20184-1-yan.y.zhao@intel.com>
 <20240507062044.20399-1-yan.y.zhao@intel.com>
 <ZktZDmcNnsHhp4Tm@infradead.org>
 <20240521154939.GH20229@nvidia.com>
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On Tue, May 21, 2024 at 12:49:39PM -0300, Jason Gunthorpe wrote:
> On Mon, May 20, 2024 at 07:07:10AM -0700, Christoph Hellwig wrote:
> > On Tue, May 07, 2024 at 02:20:44PM +0800, Yan Zhao wrote:
> > > Introduce and export interface arch_clean_nonsnoop_dma() to flush CPU
> > > caches for memory involved in non-coherent DMAs (DMAs that lack CPU cache
> > > snooping).
> > 
> > Err, no.  There should really be no exported cache manipulation macros,
> > as drivers are almost guaranteed to get this wrong.  I've added
> > Russell to the Cc list who has been extremtly vocal about this at least
> > for arm.
> 
> We could possibly move this under some IOMMU core API (ie flush and
> map, unmap and flush), the iommu APIs are non-modular so this could
> avoid the exported symbol.

Though this would be pretty difficult for unmap as we don't have the
pfns in the core code to flush. I don't think we have alot of good
options but to make iommufd & VFIO handle this directly as they have
the list of pages to flush on the unmap side. Use a namespace?

Jason