linux-kernel - Re: [PATCH 5/5] iommufd: Flush CPU caches on DMA pages in non-coherent domains

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Message-ID: <ZltQ3PyHKiQmN9SU@nvidia.com>
Date: Sat, 1 Jun 2024 13:48:28 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: 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
Subject: Re: [PATCH 5/5] iommufd: Flush CPU caches on DMA pages in
 non-coherent domains

On Mon, May 27, 2024 at 03:15:39PM +0800, Yan Zhao wrote:
> On Wed, May 22, 2024 at 02:01:50PM -0300, Jason Gunthorpe wrote:
> > On Wed, May 22, 2024 at 02:29:19PM +0800, Yan Zhao wrote:
> > > > > If you want to minimize flushes then you can't store flush
> > > > > minimization information in the pages because it isn't global to the
> > > > > pages and will not be accurate enough.
> > > > > 
> > > > > > > If pfn_reader_fill_span() does batch_from_domain() and
> > > > > > > the source domain's storage_domain is non-coherent then you can skip
> > > > > > > the flush. This is not pedantically perfect in skipping all flushes, but
> > > > > > > in practice it is probably good enough.
> > > > > 
> > > > > > We don't know whether the source storage_domain is non-coherent since
> > > > > > area->storage_domain is of "struct iommu_domain".
> > > > >  
> > > > > > Do you want to add a flag in "area", e.g. area->storage_domain_is_noncoherent,
> > > > > > and set this flag along side setting storage_domain?
> > > > > 
> > > > > Sure, that could work.
> > > > When the storage_domain is set in iopt_area_fill_domains(),
> > > >     "area->storage_domain = xa_load(&area->iopt->domains, 0);"
> > > > is there a convenient way to know the storage_domain is non-coherent?
> > > Also asking for when storage_domain is switching to an arbitrary remaining domain
> > > in iopt_unfill_domain().
> > > 
> > > And in iopt_area_unfill_domains(), after iopt_area_unmap_domain_range()
> > > of a non-coherent domain which is not the storage domain, how can we know that
> > > the domain is non-coherent?
> > 
> > Yes, it would have to keep track of hwpts in more case unfortunately
> > :(
> Current iopt keeps an xarray of domains. Could I modify the xarray to store
> hwpt instead? 

Probably yes

Jason