linux-kernel - Re: [PATCH v2 1/3] iommu/vt-d: Clear Present bit before tearing down PASID entry

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Message-ID: <CAAywjhT=QwKyh2XYJ82MQ-1Ot8Vv+8t69p8JdVTKhA=LX+c_jA@mail.gmail.com>
Date: Tue, 20 Jan 2026 10:14:03 -0800
From: Samiullah Khawaja <skhawaja@...gle.com>
To: Dmytro Maluka <dmaluka@...omium.org>
Cc: Lu Baolu <baolu.lu@...ux.intel.com>, 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>, iommu@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, 
	"Vineeth Pillai (Google)" <vineeth@...byteword.org>, Aashish Sharma <aashish@...hishsharma.net>
Subject: Re: [PATCH v2 1/3] iommu/vt-d: Clear Present bit before tearing down
 PASID entry

On Tue, Jan 20, 2026 at 5:56 AM Dmytro Maluka <dmaluka@...omium.org> wrote:
>
> On Tue, Jan 20, 2026 at 02:18:12PM +0800, Lu Baolu wrote:
> > The Intel VT-d Scalable Mode PASID table entry consists of 512 bits (64
> > bytes). When tearing down an entry, the current implementation zeros the
> > entire 64-byte structure immediately using multiple 64-bit writes.
> >
> > Since the IOMMU hardware may fetch these 64 bytes using multiple
> > internal transactions (e.g., four 128-bit bursts), updating or zeroing
> > the entire entry while it is active (P=1) risks a "torn" read. If a
> > hardware fetch occurs simultaneously with the CPU zeroing the entry, the
> > hardware could observe an inconsistent state, leading to unpredictable
> > behavior or spurious faults.
> >
> > Follow the "Guidance to Software for Invalidations" in the VT-d spec
> > (Section 6.5.3.3) by implementing the recommended ownership handshake:
> >
> > 1. Clear only the 'Present' (P) bit of the PASID entry.
> > 2. Use a dma_wmb() to ensure the cleared bit is visible to hardware
> >    before proceeding.
> > 3. Execute the required invalidation sequence (PASID cache, IOTLB, and
> >    Device-TLB flush) to ensure the hardware has released all cached
> >    references.
> > 4. Only after the flushes are complete, zero out the remaining fields
> >    of the PASID entry.
> >
> > Also, add a dma_wmb() in pasid_set_present() to ensure that all other
> > fields of the PASID entry are visible to the hardware before the Present
> > bit is set.
> >
> > Fixes: 0bbeb01a4faf ("iommu/vt-d: Manage scalalble mode PASID tables")
> > Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
>
> Reviewed-by: Dmytro Maluka <dmaluka@...omium.org>
>
> > ---
> >  drivers/iommu/intel/pasid.h | 14 ++++++++++++++
> >  drivers/iommu/intel/pasid.c |  6 +++++-
> >  2 files changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/iommu/intel/pasid.h b/drivers/iommu/intel/pasid.h
> > index b4c85242dc79..0b303bd0b0c1 100644
> > --- a/drivers/iommu/intel/pasid.h
> > +++ b/drivers/iommu/intel/pasid.h
> > @@ -234,9 +234,23 @@ static inline void pasid_set_wpe(struct pasid_entry *pe)
> >   */
> >  static inline void pasid_set_present(struct pasid_entry *pe)
> >  {
> > +     dma_wmb();
> >       pasid_set_bits(&pe->val[0], 1 << 0, 1);
> >  }
> >
> > +/*
> > + * Clear the Present (P) bit (bit 0) of a scalable-mode PASID table entry.
> > + * This initiates the transition of the entry's ownership from hardware
> > + * to software. The caller is responsible for fulfilling the invalidation
> > + * handshake recommended by the VT-d spec, Section 6.5.3.3 (Guidance to
> > + * Software for Invalidations).
> > + */
> > +static inline void pasid_clear_present(struct pasid_entry *pe)
> > +{
> > +     pasid_set_bits(&pe->val[0], 1 << 0, 0);
> > +     dma_wmb();
> > +}
> > +
> >  /*
> >   * Setup Page Walk Snoop bit (Bit 87) of a scalable mode PASID
> >   * entry.
> > diff --git a/drivers/iommu/intel/pasid.c b/drivers/iommu/intel/pasid.c
> > index 3e2255057079..eb069aefa4fa 100644
> > --- a/drivers/iommu/intel/pasid.c
> > +++ b/drivers/iommu/intel/pasid.c
> > @@ -272,7 +272,7 @@ void intel_pasid_tear_down_entry(struct intel_iommu *iommu, struct device *dev,
> >
> >       did = pasid_get_domain_id(pte);
> >       pgtt = pasid_pte_get_pgtt(pte);
> > -     intel_pasid_clear_entry(dev, pasid, fault_ignore);
> > +     pasid_clear_present(pte);
> >       spin_unlock(&iommu->lock);
> >
> >       if (!ecap_coherent(iommu->ecap))
> > @@ -286,6 +286,10 @@ void intel_pasid_tear_down_entry(struct intel_iommu *iommu, struct device *dev,
> >               iommu->flush.flush_iotlb(iommu, did, 0, 0, DMA_TLB_DSI_FLUSH);
> >
> >       devtlb_invalidation_with_pasid(iommu, dev, pasid);
> > +     intel_pasid_clear_entry(dev, pasid, fault_ignore);
> > +     if (!ecap_coherent(iommu->ecap))
> > +             clflush_cache_range(pte, sizeof(*pte));
> > +
> >       if (!fault_ignore)
> >               intel_iommu_drain_pasid_prq(dev, pasid);
> >  }
> > --
> > 2.43.0

Reviewed-by: Samiullah Khawaja <skhawaja@...gle.com>