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

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Date: Wed, 22 May 2024 11:41:42 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Christoph Hellwig <hch@...radead.org>, <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 01:00:16PM -0300, Jason Gunthorpe wrote:
> 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?
Given we'll rename this function to arch_flush_cache_phys() which takes physical
address as input, and there're already clflush_cache_range() and
arch_invalidate_pmem() exported with vaddr as input, is this export still good?