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

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Date: Thu, 16 May 2024 08:38:12 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: "Zhao, Yan Y" <yan.y.zhao@...el.com>, Jason Gunthorpe <jgg@...dia.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>, "alex.williamson@...hat.com"
	<alex.williamson@...hat.com>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
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	"baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>, "Liu, Yi L"
	<yi.l.liu@...el.com>
Subject: RE: [PATCH 5/5] iommufd: Flush CPU caches on DMA pages in
 non-coherent domains

> From: Zhao, Yan Y <yan.y.zhao@...el.com>
> Sent: Thursday, May 16, 2024 10:33 AM
> 
> On Wed, May 15, 2024 at 05:43:04PM -0300, Jason Gunthorpe wrote:
> > On Wed, May 15, 2024 at 03:06:36PM +0800, Yan Zhao wrote:
> >
> > > > So it has to be calculated on closer to a page by page basis (really a
> > > > span by span basis) if flushing of that span is needed based on where
> > > > the pages came from. Only pages that came from a hwpt that is
> > > > non-coherent can skip the flushing.
> > > Is area by area basis also good?
> > > Isn't an area either not mapped to any domain or mapped into all
> domains?
> >
> > Yes, this is what the span iterator turns into in the background, it
> > goes area by area to cover things.
> >
> > > But, yes, considering the limited number of non-coherent domains, it
> appears
> > > more robust and clean to always flush for non-coherent domain in
> > > iopt_area_fill_domain().
> > > It eliminates the need to decide whether to retain the area flag during a
> split.
> >
> > And flush for pin user pages, so you basically always flush because
> > you can't tell where the pages came from.
> As a summary, do you think it's good to flush in below way?
> 
> 1. in iopt_area_fill_domains(), flush before mapping a page into domains
> when
>    iopt->noncoherent_domain_cnt > 0, no matter where the page is from.
>    Record cache_flush_required in pages for unpin.
> 2. in iopt_area_fill_domain(), pass in hwpt to check domain non-coherency.
>    flush before mapping a page into a non-coherent domain, no matter where
> the
>    page is from.
>    Record cache_flush_required in pages for unpin.
> 3. in batch_unpin(), flush if pages->cache_flush_required before
>    unpin_user_pages.

so above suggests a sequence similar to vfio_type1 does?