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 10:32:43 +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 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.