linux-kernel - Re: [PATCH 3/5] x86/mm: Introduce and export interface arch_clean_nonsnoop

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Date: Tue, 21 May 2024 13:00:16 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: Yan Zhao <yan.y.zhao@...el.com>, 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,
	Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH 3/5] x86/mm: Introduce and export interface
 arch_clean_nonsnoop_dma()

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