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

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Message-ID: <aWldkApQ3C9pfZ3_@google.com>
Date: Thu, 15 Jan 2026 22:35:12 +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 Thu, Jan 15, 2026 at 10:45:12AM +0800, Baolu Lu wrote:
> On 1/14/26 19:12, Dmytro Maluka wrote:
> > 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. :))
> 
> You are right. A spinlock doesn't stop the hardware. The spinlock
> serializes software threads to ensure the hardware walker always sees a
> consistent entry.
> 
> When a PASID entry is active (P=1), other kernel paths might modify
> the control bits in-place. For example:
> 
> void intel_pasid_setup_page_snoop_control(struct intel_iommu *iommu,
>                                           struct device *dev, u32 pasid)
> {
>         struct pasid_entry *pte;
>         u16 did;
> 
>         spin_lock(&iommu->lock);
>         pte = intel_pasid_get_entry(dev, pasid);
>         if (WARN_ON(!pte || !pasid_pte_is_present(pte))) {
>                 spin_unlock(&iommu->lock);
>                 return;
>         }
> 
>         pasid_set_pgsnp(pte);
>         did = pasid_get_domain_id(pte);
>         spin_unlock(&iommu->lock);
> 
>         intel_pasid_flush_present(iommu, dev, pasid, did, pte);
> }
> 
> In this case, the iommu->lock ensures that if two threads try to modify
> the same active entry, they don't interfere with each other and leave
> the entry in a 'torn' state for the IOMMU hardware to read.
> 
> In intel_pasid_tear_down_entry(), once the PASID entry is deactivated
> (setting P=0 and flushing caches), the entry is owned exclusively  by
> the teardown thread until it is re-configured. That's the reason why the
> final zeroing doesn't need the spinlock.

I see. Am I correct that those other code paths (modifying an entry
in-place) are not supposed to do that concurrently with
intel_pasid_tear_down_entry(), i.e. they should only do that while it is
guaranteed that the entry remains present? Otherwise there is a bug
(hence, for example, the WARN_ON in
intel_pasid_setup_page_snoop_control())? So, holding iommu->lock during
entry teardown is not strictly necessary (i.e. we could unlock it even
earlier than setting P=0), i.e. holding the lock until the entry is
deactivated is basically just a safety measure for possible buggy code?