linux-kernel - RE: [PATCH 01/12] iommu/vt-d: Use iommu_get_domain_for

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Message-ID: <BN9PR11MB5276CCB7D5158C028AD40FF88CDF9@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Wed, 1 Jun 2022 08:53:14 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        "Martins, Joao" <joao.m.martins@...cle.com>
CC:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Will Deacon" <will@...nel.org>, "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 01/12] iommu/vt-d: Use iommu_get_domain_for_dev() in
 debugfs

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, June 1, 2022 7:11 AM
> 
> On Tue, May 31, 2022 at 10:22:32PM +0100, Robin Murphy wrote:
> 
> > There are only 3 instances where we'll free a table while the domain is
> > live. The first is the one legitimate race condition, where two map requests
> > targeting relatively nearby PTEs both go to fill in an intermediate level of
> > table; whoever loses that race frees the table they allocated, but it was
> > never visible to anyone else so that's definitely fine. The second is if
> > we're mapping a block entry, and find that there's already a table entry
> > there, wherein we assume the table must be empty, clear the entry,
> > invalidate any walk caches, install the block entry, then free the orphaned
> > table; since we're mapping the entire IOVA range covered by that table,
> > there should be no other operations on that IOVA range attempting to walk
> > the table at the same time, so it's fine.
> 
> I saw these two in the Intel driver
> 
> > The third is effectively the inverse, if we get a block-sized unmap
> > but find a table entry rather than a block at that point (on the
> > assumption that it's de-facto allowed for a single unmap to cover
> > multiple adjacent mappings as long as it does so exactly); similarly
> > we assume that the table must be full, and no other operations
> > should be racing because we're unmapping its whole IOVA range, so we
> > remove the table entry, invalidate, and free as before.
> 
> Not sure I noticed this one though
> 
> This all it all makes sense though.

Intel driver also does this. See dma_pte_clear_level():

		/* If range covers entire pagetable, free it */
		if (start_pfn <= level_pfn &&
		     last_pfn >= level_pfn + level_size(level) - 1) {
			...