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

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Message-ID: <ZlQzGzU1/CLY0eOg@yzhao56-desk.sh.intel.com>
Date: Mon, 27 May 2024 15:15:39 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Jason Gunthorpe <jgg@...dia.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 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?