linux-kernel - Re: [PATCH 2/3] iommu/vt-d: Clear Present bit before tearing down PASID entry

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Message-ID: <aWd6KIGBiE-nL2IT@mitya-t14-2025>
Date: Wed, 14 Jan 2026 12:12:40 +0100
From: Dmytro Maluka <dmaluka@...omium.org>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Kevin Tian <kevin.tian@...el.com>, Jason Gunthorpe <jgg@...dia.com>,
	Samiullah Khawaja <skhawaja@...gle.com>, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	"Vineeth Pillai (Google)" <vineeth@...byteword.org>,
	Aashish Sharma <aashish@...hishsharma.net>
Subject: Re: [PATCH 2/3] iommu/vt-d: Clear Present bit before tearing down
 PASID entry

On Wed, Jan 14, 2026 at 01:38:13PM +0800, Baolu Lu wrote:
> On 1/14/26 03:34, Dmytro Maluka wrote:
> > On Tue, Jan 13, 2026 at 11:00:47AM +0800, Lu Baolu wrote:
> > > +	intel_pasid_clear_entry(iommu, dev, pasid, fault_ignore);
> > Is it safe to do this with iommu->lock already unlocked?
> 
> Yes, it is. The PASID entry lifecycle is serialized by the iommu_group-
> >mutex in the iommu core, which ensures that no other thread can attempt
> to allocate or setup this same PASID until intel_pasid_tear_down_entry()
> has returned.
> 
> The iommu->lock is held during the initial transition (P->0) to ensure
> atomicity against other hardware-table walkers, but once the P bit is
> cleared and the caches are flushed, the final zeroing of the 'dead'
> entry does not strictly require the spinlock because the PASID remains
> reserved in software until the function completes.

Ok. Just to understand: "other hardware-table walkers" means some
software walkers, not hardware ones? Which software walkers are those?
(I can't imagine how holding a spinlock could prevent the hardware from
walking those tables. :))