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

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Date: Wed, 22 May 2024 14:29:19 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Jason Gunthorpe <jgg@...dia.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>
Subject: Re: [PATCH 5/5] iommufd: Flush CPU caches on DMA pages in
 non-coherent domains

> > 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?